Whats everyones take on in-game time to go from level 0, whatever that looks like in your campaign, to level 1? To me level 0 is someone completely untrained in their profession. 

Unless you’re a sorcerer, it would take months at least of training and work. Usually years.

Personally I was thinking somewhere in the range of 6 months. If you use the US military as a comparison then basic, and your A school, what ever it be in, usually lasts about 6 months give or take your job. I remember in one of the Drizzt books where Catti went off to start being a wizard she was told she would be casting cantrips and basic spells in a matter of months, I could be wrong on that though. 

I hate to say "it depends" but.... well.... it depends. Mostly it depends on your character class.

Barbarian: so you're a random spear sharpener with dreams of killing big things, so you train your body and you train with your weapons and you go on a bunch of raids and skirmishes with the real warriors in your tribe. But the physical training is just half of it. You also have to learn to harness your rage. For that you need to sit at the feet of the elders and listen to their stories, and drink to celebrate their exploits, and you need to fight... a lot. And then some more fighting. Then, eventually, during one of those skirmishes with a neighboring tribe your buddy right next to you takes a spear through the neck and you - just - SNAP! You Rage! Congrats, you just made 1st level barbarian. Elapsed time: 4 to 6 months. 

Bard: assuming you were born with a good ear and a natural charisma, it'll still require intensive training to truly master your instrument. Years of study and practice and gigs that pay you in leftover chicken and stale beer. Plus you need to learn all the tales and epics and odes that not only entertain but inspire. That means extensive study in dusty libraries but also many nights in taverns and old soldier homes buying drinks and bending ears. It's hard work making people happy! Elapsed time: 2 to 3 years.

Cleric: Now it gets tricky. Deities require as much from their followers as they give them. So you'll not only need to spend time at temple learning the lore of the faith, and practicing the rituals of the faith, you'll also need to prove to the deity that you're worthy to carry their banner. You're not just an employee, you literally represent that deity personally. So the standards will likely be much higher. Learning the craft is half the battle, if that. Proving yourself may take much longer. Elapsed time: 3 to 5 years.

Druid: Nature is tricky. So maybe you were born a wild child and just take to nature, well.... naturally. (sorry). but trouncing around in the forest and streams reveling in the beauty of nature is one thing. Harnessing the powers of nature requires you to find that inner stillness. Again, learning the craft is one thing, but getting the beasts and the trees and the elements to trust you - that's tough. Elapsed time: 2 to 6 years. 

Fighter: Now this is much more straightforward. Exercise, train, fight, exercise, train, fight, exercise, train, fight. Whether you train at a military academy, or whether you train by defending the family farm from orcs, it's all up to you. No one's gonna give it to you. Just do the work and see the results. Elapsed time: 4 to 6 months.

Monk: Before you can even begin your training you'll need to find a trainer or a temple. So you may have to travel and search. Then you'll have to prove to them that you deserve to be trained. That could mean a long time spent cleaning and cooking and farming, to instill a work ethic and a sense of discipline and responsibility. By the time you finally begin your training you'll realize that you have only just begun what will be a very long and grueling journey. Elapsed time: 3 to 8 years.

Paladin: Basically Fighter plus Cleric. Elapsed time: 4 to 6 years.

Ranger: Basically Druid plus Fighter, with extensive time spent wandering the wilderness thrown in. Elapsed time: 3 to 7 years.

Rogue: This gets tricky because rogue is one of just a few classes that you might start training for at a very young age. But aside from just learning how to pick locks, and sneak past guards, and forge handwriting, and do acrobatics, you also have to earn the trust of the people you work with. Loose lips sink thieves guilds. Elapsed time: 1 to 5 years.

Sorcerer: It's in your blood. You were born that way. Maybe the powers were evident since birth. Maybe it's like X-Men and they only become apparent during a very high stress incident in puberty. Either way... you're a sorcerer. Congrats. Elapsed time: zero.

Warlock: This gets really tricky. this could be similar to either the Sorcerer or the Cleric. Maybe your patron requires you to memorize their lore before granting you their powers. Maybe your patron requires you to do some totally unrealistic thing before granting you their powers. Or maybe your patron just drops it on you and walks away laughing. Who knows. Elapsed time: maybe zero, maybe a year, maybe 20 years. Seriously. It's anybody's guess.

Wizard: Learning to cast spells in likely equivalent to getting a PhD in some ridiculous field like particle physics or quantum whatever something. Study, memorization, training, ad nauseam. Elapsed time: 6 to 12 years. 

These are guesses. Just my 2 c.p.

Do you actually play level 0 to 1 in game, or do you just want to create a realistic timeline for backstory?

It depends on the class. A barbarian and a sorcerer are born that way. Although a barbarian might reach level one in a coming-of-age ritual. A sorcerer might develop wild magic in puberty. They might meet and have a hilarious odd-couple sitcom.

I think it would br interesting to play a session or two at level 0. Like you roll your first hit die instead of taking the max. You could start with 1 hit point. And you're not proficient with any weapons. And you've started wizard school but you"re the last student who hasn't performed their first cantrip. How do you deal with problems as an ordinary person who expects to lose any fight they pick. Maybe with more shrewdness and tact.

How do you see it taking 6-12 years to be a level 1 wizard? we are talking about level 1 not 20. After 1-2 semesters a dedicated philosophy student (im speaking from experience) is doing to have a wide and decent grasp of topics and concepts and will be able to start forming basic concepts of their own. A level 1 wizard isnt a master of anything, they are very much an apprentice, a novice. This holds true for most of the other classes except for maybe a warlock and sorcerer as well a warlock is a cheater and a sorcerer the savant. 

So maybe my initial 6-ish month estimate should be seen as either militaristic A school training or 2 semesters of full time degree specific college work. 

You've completed two semesters of philosophy.

Cool.

What spells can you cast?

The players handbook says that ‘not every _____ is a ______’ in pretty much every class description. To me, the difference between a player and, let’s say, a veteran NPC, is that spark of destiny. 

All level 1 player classes using a standard array are better than the Commoner stat block. (I use standard array in this example because, as I mentioned many times, I managed to roll 3 sets of 4 ones with dice-generation using the same method and dice as everyone else.) This is important because level 1 classes are better than commoners.

If a character comes from a Commoner, then yes. I see some need to have a Level 0 progression, but as mentioned, that can be handled in backstory. Still, it might useful to have some suggestions and examples from WotC on how to go from Commoner to Level 1 player class. The suggestions in XGtE can get a bit too in-depth for a player character at level 1, too veteran-y much of the time and better for starting at higher levels, 3 at minimum.

There's a YouTuber who has an episode on the minimum recommendations for a backstory to get from an ordinary (and mostly happy) commoner to a level 1 player class adventurer through backstory. I'll link it here only because it's directly relevant to this conversation but is not intended to be an advertisement for that channel.

In my reckoning, this is a level 0 Fighter. I get the party from 1st—2nd level on 1 session, from 2nd—3rd in another 2 sessions, an then up to 5th level in the next 6-8 sessions. For me, getting from level-0 to 1st level would take an ‘80s movie training montage.

The Survivor rules in Van Richten's Guide could be used for a level 0 adventure.

I have toyed with the idea of creating a level 0 one-shot which would be built around character building. The idea would be to have a series of tests which would determine which skills are in which position in a standard-array style character making. 

The reason I stopped was that this means the players won't be able to make character backgrounds or determine their class before the session. That means that the player who wanted to play a sorcerer might end up with barbarian stats to work with, because they rolled high on strength and constitution and low on intelligence and wisdom. That woulsn't be fun for anyone.

As for how long it takes for someone to go from level 0 to level 1, it could be anything. A fighter might have been a guard for years before suddenly realising he's better at fighting than everyone else, because there had been no fights.

A Human Commoner has an ability score deficiency of 12 compared to the Standard Array and a bonus deficiency of 5 compared to the Standard Array.

That would take quite a long time to go from Level 0 as a Commoner to Level 1 as an adventurer with a Standard Array.

I use pretty strict training needs between levels without including level 0. At the time you start a great story together, there's already a good reason to be level 1.

Players who like doing backstories might get a bit dis-engaged, although I completely get the idea of experiencing the full growth from a mere peasant to the hero of the realm. I think there are other means, depending on your style of campaign. E.g. a level 1 character is a force to be reckoned with - in the local tavern. Depending on how you define the grimness and difficulty of your world, the journey from one level to another can feel like a breeze or a minor miracle.