For me it's within the first few hours. If I hated the first two hours of a game then I'm probably going to hate the next 20.
Case in point: L.A. Noire. The first few hours bored me to death but I thought maybe if I stick with the game it would get better. Twenty shitgargling hours later as the credits were rolling I swore I'd never put myself through that again.
Of course some people think it's only fair to judge a game after you've completed it 100%. Some people also think the Holocaust didn't happen and that God put dinosaur bones n the ground to fuck with us.
Coincidence? Yes, probably.