Hoping you have some advice on how to handle Suffocating in Combat/Holding breath in combat 

Situation is this :- I'm throwing a Dybbuk at my players it has the spell Phantasmal Force which I'm intending to use that it is giving the character the illusion that he is covered in demonic slime choking him suffocating him.

Rules :- Suffocating (A creature can hold its breath for a number of minutes equal to 1 + its Constitution modifier (minimum of 30 seconds)) 
So this players character has CON 14 so a (+2) modifier for a total of 3
character can hold breath for 3 min  
1 round =  6-second span 
so 3 min = 30 rounds

I don't know about you but I've not ever had a 30 round fight, while the idea of having to fight holding your breath and suffocating seems like a ticking time bomb it just doesn't seem to translate well....

Any suggestions? am I doing something wrong?

Thanks for any replies thoughts

Holding your breath is different than suffocating or choking. 

At my table, during combat, I run holding your breath like concentrating on a spell. If you take damage, you make a con save DC = half damage taken or 10, whichever is higher. Succeed, you hold your breath. Fail, you loose 1 minute of air. If you cast a spell requiring a verbal component, you willingly loose 1 minute of air. You run out of air, you are suffocating.

Once suffocating, they have their Con mod. in rounds to find air or drop to 0 hp, and start making death saves. Which, even if they succeed their death saves, they die, can't regain hit points without air.

If the fight starts on dry land, and winds up in the water, or with someone being choked, like Ettercap (variant), they make a wisdom save. I scale the result from DC 15 in increments of 5. Base success (15) gets 30 seconds of air, with each order of success/failure equal to 30 seconds each. Roll a 10 or lower, you are suffocating. Roll a 25, you have 90 seconds.

Oh thank you that's super useful advice Kaavel
Yeah i didn't intend to equate suffocating and choking was intended to describe what's happening or rather what the Phantasmal Force is giving the character the illusion of what is happening... I guess like the struggle you see in some particularly grizzly thriller where the hero has to fight while some one is covering their face with a plastic bag... except in this case its demonic slime and its the entire body restraining the hero's movement sight and hearing... err but in his head

Anyways super cool advice I think I'll start having him do the wisdom save DC15 to find characters "air time?" and then do the con DC10 save on the phantasm's 1d6 psychic damage... (calling it physical or necrotic don't want to give the game away) on fail lose 1 min of air... when "air time<or= 0" he has 2 rounds before 0 hp unconscious but no actual real death save as it is an illusion... though I think I'll make the player do them (evil DM laughter) 

think I've understood your advice correctly thanks again that's a real ticking time bomb of a combat situation I was hoping for 

UPdate on how it went...

Darn it Typical! the players character managed to get a lucky hit and brake the Dybbuk's concentration on the Phantasmal Force Ahhh! players always ruining evil DM plans.... lol was super though rather shorter then hoped for he managed to get a hit practically straight away though it did have him sweating for a little bit :D

what I might change :- given it was a individual situation and I have a small group I'd not go with the 5 increment success for getting "air time" could have just gone with increments of 1 ie 6 sec or one round, roll a 18 you have 8 rounds or 48 sec   
don't get me wrong 5 increments success is nice and stream lined particularly if you have several characters in the same "turned over" boat

Thanks again Kaavel was super advice 

Glad I could help. Sorry you couldn't use it to full effect. 

Hint: Crocodiles love to drown their prey.

I run it a couple of ways, depending on the situation at hand.

1) holding your breath outside of combat, like swimming or something, same as RAW

2) In combat, but holding breath underwater/or in poison gas, CON saves on being hit(like a concentration check), or lose 30 seconds of air

3) Being actually strangled: 1 + Con Modifier rounds. This is fun to have an ogre pick up a spellcaster and run away with them in some theatre of the mind combat, lots of pressure to both catch up and free the caster.  This is what I would use for your demonic sludge.

The best house rule for this is that the normal breath holding rules only apply outside of combat since naturally it's harder to do it when getting hit. Treat it like concentration where any damage forces a player to make a DC10 Constitution check. If they fail, then suffocation rules apply.

For phantasmal force specifically. I would simply have it do 1d6 psychic damage a round and describe it as them trying to hold their breath so they can't speak or use verbal spell components.  I'd be hesitant to give phantasmal force more power than that because it's a player option.  Since its an illusion its not really necessary to act as if they had to hold their breath for a bit they can merely feel like they've already run out and take damage immediately. 

Generally the suffocating rules are not designed so that you can kill something with suffocation in combat instead its designed so that you can swim a bit, get into a fight and then swim back to the surface because if you are downed by suffocation the effects are worse than being reduced to 0 normally. 

A common house rule is that you only get to hold your breath if you chose to go under water making drowning offensively far easier but Id be careful with that because suffocation is a kind of instant kill effect.

thx for the replys Solarsyphon Cdub808 and TONEY_BALONEY999 yeah I wasn't intending to kill off my players character and I used the 1d6 though I didn't flag it as psychic dmg and I wasn't intending on making him do death saves when unconscious... well yes I was but not real ones.... evil laughter 

My ruling is that if something/someone is actively choking a creature, then that creature skips past the “holding you breath” stage and goes right to the “choking” stage. With a Con of 16 the creature will hit unconscious in 3 rounds (18 seconds), which is pretty realistic.

thanks for the reply IamSposta hope your not suffering to much of a deep freeze "bomb cyclone" (S/E Pennsylvania) and have good hol's
do I under stand correctly you go for no rolls?
6 sec (1 round) minimum and then just look at character CON with CON over 14 getting 6sec (1 round extra) per CON ?

wishing you all the best thanks again for the reply

edit yeah 6sec per round not 3sec was a typeO :o

Thank you. Yeah, I just use their Con modifier to see how long they can withstand the choking. One minor correction, a round is 6 seconds, not 3. I usually use the rolls to see if they can escape the grapple or whatever it is that’s choking them instead. So far it’s usually been a grapple, so I just rely on the rules for escaping them to determine when they are no longer choking. In the case of your slime monster, I’m not entirely sure how I would handle it. What was the DC for escape? I’m assuming the caster’s Spell Save DC. Yes?

Corrected that typeO I mean 6sec per round... the Spell Save DC was 12 though in the end my player managed to get a rather lucky hit in while blinded and brake the concentration of the Dybbuk