I am in the middle of writing a campaign for 1st to 7th level characters. How should I get them all the way the 7th level without making it really long? It is a role play heavy campaign and I am not sure if I should do story milestone level up or if I should do experience level up. The final boss monster is going to be a Drow Priestess of Lolth with some minions. I'm not sure if 7th level is enough to deal with these threats. Basically, how do i get the characters to that level without having a really long campaign?

Don’t plan by level plan by sessions - you clearly have a length in mind so if you know how regularly you want to play give it a round number you are happy with

you can plan around that with an increase in time to go up the levels but 

session 1 : level 1

session 2-3 : level 2

session 4-5 : level 3

session 6-8 : level 4

session 9-11 : level 5

Session 12-15 : level 6

Session 16-19 : level 7

Session 20 final session

you would be looking at 20 weeks so about 4-5 months.

Thank you for the response sardonicmonkey! I don't have a specific time in mind right now but this definitely helped!

It sounds like you're viewing the whole campaign as just some stuff that happens that allows the PCs to be high enough level to do the fight you actually want to do. If that's really the case, run a level 7 one-shot.

But if you actually want to make a fun campaign with adventures and NPCs and treasure, then a "really long campaign" is exactly what you want, because it's not just about the end battle. The whole thing should be fun. The whole thing should be stuff you want to do.

I agree with scaterbraind in that if you want them to have a boss fight at a certain level, start there (or close) to make it brief.  You mentioned you wanted it to be RP heavy, so I think you're going to want a fairly long campaign.  MY suggestion, to keep it fresh and interesting is to have milestones where the party can have a big win, at different intervals.   These can be part of the milestone leveling and should be, but a little more impactful on their progress, like sub-boss type things in video games, but these would make sense in your overall story.  Important victories, recovering hostages, saving a town, outing a nefarious scoundrel, as a DM make them feel like they mean something big.  It keeps the players chomping at the bit for more and that, in turn (for me, at least) inspires the DM to work on more, better and deeper encounters and challenges.

Pre-planning a whole campaign is very different to session-by-session planning. You need to write a lot more stuff, so that if the PCs go off on a tangent, you have it all ready.

To go specifically from levels 1 to 7, use Milestone Levelling, and work out what they achieve Milestones for. Then create areas, populate them with creatures, NPCs, events, dungeons etc, as well as one-off-events that occur if the PCs are present. An outline might look like this:

• Area 1, Starting area: Small village. A variety of friendly NPCs. They have a simple task for the PCs to undertake, e.g. the classic go to the old mine and kick the goblins out. On doing this, the PCs hit the milestone and reach level 2. In the mine, they also learn that the goblins have links to larger problems in the region.
• Areas 2 and 3 can be tackled in either order. There are two smaller NPC hubs (e.g. one is a hunting lodge, the other is a roadside tavern) which serve as information centres. The PCs are a bit more out in the wilds now. Whichever they go to, there are 3 different things to do/quest ideas, e.g. stop the Owlbears, locate the lost child, work out why the river has turned green. All 3 of these things lead to a single larger event, e.g. go to the ritual point and stop the Drow sacificing someone. The PCs can do this major event without doing all 3 of the others, but ideally need clues from 2 or 3 of them to get there. When they do that, they hit a Milestone again and level up.
• Area 4: The PCs can get here from 2, or 3, but will be a little underprepared if they haven't done both. Have a friendly NPC ready to guide them back, or if they really want to press on, they can. They are level 4 when they reach here. The events here, after some poking around, lead to a long dungeon that takes several sessions to complete. Having done so, they get a Milestone and reach level 5.
• Area 5: The PCs reach a major urban centre - a small town. They probably need to race here with key information they learned in the dungeon from Area 4. This lets them buy supplies, get better armour etc. and there should be a number of things to do here, enough that the PCs feel they have freedom to go where they want. Eventually they make it down into the sewers, which are effectively a Drow themed dungeon where the sewers connect to the underdark. When they solve the town's issue, the PCs get a Milestone.
• Area 6: The PCs have located the main location for the enemy from whatever they learned in Area 5. They need to choose to head to the enemy base, preferably with time running out before something bad happens. Make sure that this takes them through some exciting terrain, and is filled with challenges. This can be a pretty linear road to follow. Let them use things they discovered earlier in the campaign. You need to throw enough challenges at them that they struggle with this part; it should feel hard. Just reaching the enemy fortress/caves/palace whatever should earn them a milestone.
• Area 7: The PCs have hit level 7 just before they got here. Give them a 3 floor dungeon that really tests them. The BBEG is at the end of the dungeon.