Yeah, opinion varies pretty wildly on when to surrender. I play with one friend who has begged his teammates not to surrender when the enemy team had 5x as many kills and we had no towers. I used to play with another guy who would stop trying as soon as one of his teammates did something he thought was stupid or there was a major team fight loss. I avoid playing with him anymore.
Really, I think I'm above average optimistic. My team has to suffer at least two crushing defeats, with no reason to believe that things will change. When there's no one on your team who can approach the enemy without getting instantly nuked, there's really no reason to continue. It usually doesn't take long before a losing battle reaches this point. But if we're behind a couple towers and come away from team fights with 1 or 2 less living players than the enemy, I'll keep at it. Hell, if we're losing team fights horribly and are way behind, but they leave themselves open for backdooring, I'll keep at it. I'm usually one of the last people to want to surrender.
Often enough, though, there will be those games where you have two leavers or really bad character match-ups, and you're just waiting 10 minutes or more for the ability to surrender. Or you'll have one teammate who's like "But I have a positive score! Don't surrender!" and another who's like "This is my last game for the night, I might as well play it out," when your team ratios are 1-3, you're 4 towers behind, and the other team decides to drag things out intentionally for 20 min. This kind of stuff happens often enough that I consider it a serious detriment to the game. Raaaage.
I'm kind of bitching because I quite the game again two days ago, but then my wife broke her toe lastnight. So I reinstalled the game... again... and played with her to help take her mind off the pain. We proceeded to have two really fun games in a row. Goddamnit. They dragged me back in.