Near the beginning of the game (Test Chamber 02 to be precise), right before you find the single-portal gun, you fall down a shaft and into some water. Wheatley asks
Can you see the portal gun?
He then continues:
Also, are you alive? That's important, should have asked that first.
I'm--do you know what I'm going to do? I'm going to work on the assumption that you're still alive and I'm just going to wait for you up ahead.
I'll wait--I'll wait one hour. Then I'll come back and, assuming I can locate your dead body, I'll bury you. Alright? Brilliant! Go team! See you in an hour! Hopefully! If you're not... dead.
- Portal Wiki
I didn't really have the time, but if you actually stand there for an hour, will he come? I realize this is pushing the bounds of game development, but it is Valve we're talking about here.