A Session's Worth

So, what's "a session's worth" of stuff?

Now, we all know that one trip to the shield-polisher with the funny accent could end up as a session's worth, and, hey, if so, excellent. But assuming--as I do when writing and adventure--that the PCs are just gonna burn through every obstacle and smell no roses, how much do you need just to get through a day?

I pretty much do things a day at a time--throw enough down to keep the girls busy for a night, probably leaving some danglers, repeat the next week. So I'm often wondering: what's the minimum I can do today and still have D&D in every direction and no improvising. (Not that mind improvising, but I like scheming more.)

I threw together a dungeon (all crawl, no NPCs) a few weeks ago on an hour-and-a-half notice that, barring any outbreaks of outside-the-box genius, would have to break down to something like: 1 minor mystery room, 1 or 2 simple, shortish fights (party split two ways and did both of them), 1 major mystery room, 2 fights of varying degrees of complexity, one with tricks thrown in. I don't remember the layout and it's too hot in LA today to get up and look but I'm guessing it was 12-15 rooms.

So--assumign you want to plan everything ahead, and if you figure you might get an all rock no talk day, how much do you figure is a session's worth of material?