Time is often said to be extremely important in an OSR game. Spell durations, movement rates, durations of torches and time between rests. But how openly do you inform them?
While you could keep time, for example, from the time it takes a torch to burn down, bear in mind that the characters are hundreds of meters underground, in complete darkness except where local light exists. They would have no concept of the passage of time, except by very specific, known measurements (like the aforementioned torch).
When I run the game, I check off turns taken as we play. 1 turn to search a room, a battle always takes 1 turn (includes time to loot the bodies and bandage injuries), for simplicity, I usually let them move 2 "units" per turn in known territory. (A unit is either a stretch of corridor or a room).
This assumes cautious movement, in areas already cleared. Characters in a hurry could probably move 3 units and while fleeing, who knows where you'll end up?
As torches burn out, or the need for rest arises, I let the players know, but otherwise, they won't know how much time has passed, unless they return to the surface or acquire some sort of time keeping device, no doubt at significant cost.
Likewise, the players should not be completely confident in the duration of spells, unless they take great care to keep track. A particularly devious DM may vary things up by a D6 roll for anything with a known duration (spell, torch, time before rest) with a 1 shortening the interval by 1 turn, and a 6 lengthening it by 1 turn)
How much focus do you put on time in your games?