How long do subdued NPCs remain unconscious for?

They do not wake up after being subdued unless found and woken up by another NPC. I don't have a linkable citation but this is stated in one of the series of tips that is displayed in the corner of the screen during mission load screens.

Once an enemy has been subdued they will stay subdued indefinitely. Unless found by one of the AI in the game they cannot be woken up. If you want to prevent an enemy from being woken up then hide the body in a box or closet. Also if he saw you perform any illegal action before being subdued then he will retain that information.

Even after being subdued if he is woken up he can tell the guards and get you in trouble unless you're wearing a different disguise. If you play the game's tutorial it tells you about hiding bodies. Anyway I hope this helps and I wish you luck. (I found that the mission in Colorado to be the hardest)