Generally, if the team counts are even, I don't surrender unless at least one inhibitor is down. Before that, the game can go either way. There are some rare exceptions, but you get the idea.
I get really sick of teammates who surrender when the game's practically even. Imagine that the score is 18-20, none of your inner turrets are down, and your 4 teammates vote yes. I've had that type of scenario play out more than a few times.
Just a few days ago, I was playing Twisted Treeline and my teammates were whining about me voting no, no chance of winning this, blah blah blah. 15 minutes later, I carried the game into a victory. A game or two later, the same thing happened, except that both of my teammates went AFK at fountain because I voted no, so yeah, instant loss being 1v3.
It seems to be a side effect of this childish mentality of "If I'm not 18-0 and showing everyone how awesome I am, then the game is not worth playing, so I'm just going to surrender ASAP to get onto the next game and get lucky there." Notice that it's those with the worst k/d ratios that are typically first to surrender, even if the game as a whole is relatively even.