Mentioned in Chapter 8: Adventuring, the main rule to travel states that a fast speed is 400 feet per minute, a normal speed is 300 feet per minute, and a slow speed is 200 feet per minute. The default, therefore, should be 300 feet per minute, particularly when there's nothing interesting (featureless corridors, etc), and 200 feet per minute when there's any particular need to be cautious (e.g. stealthy), or if they're actually looking for something.
A more detailed blog post (does not quote official material, but looks legit) suggests 5 minutes for each trap/lock if proficient and equipped, 10 minutes otherwise, up to 20 minutes for secret doors/passageways (with an Investigation (Intelligence) check to reduce time), and a general guideline of not more than 10 minutes per room for activities, unless the players choose to exhaustively search a room (20 minutes). I can't seem to find rules for these actions right now, but I remember using similar guidelines in many groups both as players and as DM.
You might want to keep track of seemingly mundane details to help calculate time. Did they spend 30 minutes deciding if they wanted mutton or pork? Did they decide to spend an hour reading books in a library? Not every room is going to go at full speed, and you might even automatically add in some time-wasters to help pad out the time if your players don't do much on their own. Add a lot of locked doors, and traps, and secret passageways, and treasure boxes. These all take time, and if designed well, will take more in-game time than real time.
The rules are a decent start to tell how much time has passed, but you should be prepared to arbitrate various random activities that do consume a lot of time but are not covered by the rules. I had one DM that even advanced time if we took too long deciding what to do in a room, as if our own indecisiveness was reflected in our characters. Be creative, and you can easily find places to realistically add many wasted hours in a day to help your players reach legitimate Long Rest points (preferably near the end of a game session).