Probably Einhander on the ps1.
I spent hours after hours on that thing.

Dirt 2 - about 20 hours

Halo 3 demo, 4 hours =)

uh uncharted 2 hands down....or do betas not count? if it does at least 30 hours plus, if not then infamous, maybe hour and half.

heavy rain demo i played it almost 40 times

The Halo 3 beta. I spent alot of time playing that when it came out

Uncharted 2 (MP): A good 20 hours.
God of War III: Perhaps 5 hours?
TFU: Something like 4 hours.

I've never timed, to be honest.

Command and Conquer (the first one).

I played those missions so many times... Dozens of hours for sure, maybe more than a hundred...

I'll just play the demo once to see if I like it and usually just wait for the full game to come out.

But you've never played a demo that's so fun that you want to play it over and over again?

And wouldn't you agree that if a demo can make suck up so much of your time, the full game probably is worth a buy?