D&D 5E Running several combats in a row

[aco175]

Is it totally unreasonable to allow short rests every three or four waves? It might take some time for the baddies to muster up the numbers for another assault, so, it's not totally unreasonable that short rests might be possible. After all, short rests do allow baddies to heal as well - don't forget that each wave does not have to fight to the death every time. Waves can come in, fall back after losing about 1/3 of their HP (I mean total for the group), regroup and add in some extra baddies to fill out the fallen, attack again.

The same baddies could potentially show up two or three times.

So, after the first wave of three or four assaults, the baddies fall back and give the PC's enough time for both sides to short rest. Wash rinse and repeat as needed.

There could also be a rotation weather on offensive or defensive. Like you say, everyone pulls back to regroup and after an hour or two fighting starts again. Depending on the types of forces, there could be breaks to gather wounded and pray even.

 

[Yaarel]

Consider redefining the Short Rest as 15 minutes. This is long enough time to justify a sense of refresh, while short enough to not feel stupid in the midst of a combat onslaught.

If a DM is utilizing combat to intentionally drain resources, remember, even 15 minutes can be interrupted if the DM wishes.

Players can continue enjoying lunch and doing light tasks after 15 minutes. They can still be taking a breaking narratively, and perhaps certain efforts gain advantage if doing so when refreshed.

The 15 minutes is mostly about narrative.

 

[Plaguescarred]

Just be mindful of the Challenge Rating and overall difficulty later waves will have because the party will be diminished, running lower on HP, spell slots, features etc that they normally have when 100%.

If you see things are getting too ugly, don't hesitate to have enemies opt for different actions than Attack in their statblock, for exemple Dash or Desingage to move to other location, spread attacks instead of focus fire, retreat, freeze in place, Dodge, Help another, Search for something, make Skill checks to Influence, Study something, Utilize item, Ready or Hide etc...

 

[DEFCON 1]

Multi-wave encounters are also usually a great time to bring out whatever "minion" rules you might use-- whether that's 4E style where every creature has just 1 hit point... or handfuls of very low-HP creatures thrown amongst the "hero" monsters. Especially useful if you have casters with AoE spells they never get to wipe out huge swathes of enemies at once with, or martials with Cleave-like abilities that can actually one-shot an enemy and pass through the attack to the next (or abilities that allow you to attack everyone adjacent to the PC for instance.)

Having a wave of like 30 enemies on the board rushing the characters-- but all of whom can get mowed down with single attacks in like one round or two-- helps create that "multi-wave effect" while not raising the deadliness to uncontrollable levels (unless of course the players DON'T use their abilities to mow them all down and then they get zerg-rushed.)