Agree even before the clothes come off the boundaries of the evening. Like say you have an 8 o'clock class, even if you don't. Or you've got to get home early because your brother needs to borrow the car. Or you've go to go to work.
(BTW, I would use all of these excuses at once!)
The point if, the guy knows in advance that he's not staying much beyond sunrise, so make the most of the the time you've got.
If the sex is great, call him back for a date on a night when your next day's schedule is more "open" and maybe you can spend some more quality time snogging and snuggling in the mutual afterglow.
I never want to spend the next day with one-night stands. The whole point of the encounter is it's a cute guy you don't know, but whom you hope is sexy and sweet. (And showered and wears clean underwear....I could go on for days!)