Also, the more of the dungeon that makes up a single scene, the more old school resource management you get.
If every room is a scene, things can be very fast and the PCs will largely be at full strength for each one. If the whole dungeon is a scene, they'll tend to go slower and hopefully worry more about ehat's behind every door and around every corner.
I get the sense OP is running something akin to world's largest dungeon, or temple of elemental evil. If that's the case, then I think Krenksy's recommendation is the best. Basically, you should have scene breaks as often as you need to in order to keep the resource management at the level you want it to be. Don't worry about time so much. It was my experience with World's Largest Dungeon that time becomes fairly meaningless, and the party will pretty much just go until they no longer feel certain they can survive the next room. They they decide to stop and backtrack to somewhere they can hole up and sleep to refresh their spells and whatnot. So, basically the more often you have new scenes the longer the party will feel comfortable going before they stop and rest.
The way I would recommend doing that at first is to just stretch scenes until the players become reluctant to advance or start looking for a place to rest. Then you have a scene break and they should go back to being fairly confident. Play with the timing with that until you get a good feel for how long a scene should be with your campaign and players. Once you've got a good feel for how long a scene should be, you could then experiment with constructing the narrative events to make for good scene breaks.
Anyway, that's just my thoughts on the matter.