Now that I've had some sleep, I can provide you with a bit more context.
First, here is a map of the adventure location:
The PCs are being held in separate cells in Area 20. Their gear is being kept in Area 24, pending distribution.
The fastest way out would be via Area 18, which leads up to the corridor to the left of Area 12a, which leads into the shrine (Area 7) and out through the front door (Area 1). However, as you can see on the map, there is a trap in the corridor (it's a portcullis cage trap), so they could easily find themselves caught again if they go that way (not that they know that).
The steeders (giant spider mounts) are kept in Area 21. The existing slaves (24 human and halfling commoners) are put to work, a dozen at a time, in the mine (Area 22). The rest of the time, they are kept in their cells. There were more slaves at one stage, but the duergar have worked them to death (the PCs found the mass grave in the woods at the base of the hill - see the environs map).
Second, I'll concede that the PCs could potentially get a short rest. The only reason they're still at 1 hp really is because only a few hours have passed since they were captured (
"A stable creature that isn't healed regains 1 hit point after 1d4 hours"). Also, one of the PCs actually still has most of her hit points because she was only knocked unconscious by drow poison rather than dropped to 0 hp through damage. That lasts for 1 hour. She's a rogue, as well, so she's probably got the best chance of escaping on her own aside from the dragonborn barbarian, who has advantage on Str checks to break objects thanks to his Aspect of the Bear trait.
So yeah, not a lot of time has passed yet since they were captured. The duergar will certainly feed them and take reasonably good care of them given enough time. The PCs did a real number on the duergar forces, though, so they'll be operating a skeleton crew for the next few days at least - until they can send for and receive reinforcements from the nearest duergar settlement. They'll probably have to suspend the mining operations, as they won't have enough men to guard both the prisoners and the fortress above at the same time. They won't be able to escort the PCs to an Underdark slave market until reinforcements arrive either, so for now the PCs can sit pretty in their prison cells. Unless they summon in some devils as reinforcements (the leader is a half-fiend durzagon in league with a pit fiend spirit) ...
One thing that occurred to me this morning, though, is that the duergar are going to want to interrogate the PCs. They'll want to know who attacked them and why and what sort of abilities these people have - so they can better gauge how much they'll be worth as slaves, if nothing else. So I'll have to set up interrogation scenes. Given time, the duergar clerics can always prep and cast
zone of truth. The warlock can also cast
mass suggestion to get them to cooperate.