Many choices are timed in The Walking Dead, and you can always tell. Except for this one decision, and it happens to be the most gut-wrenching choice you have to make near the story's conclusion. Warning: major spoilers ahead.
That's when you send Clementine off and you have to decide whether or not she kills you.
According to an interview with Giant Bomb, Telltale says that if you take too long to decide, Clemetine will decide for you using what you've taught her along the way.
This is a timed choice, and if you let the timer run out and Lee says nothing, Clementine does decide for herself. If you aren't there for Clementine when she asks you what you should do, she does actually look back at everything that's happened and she'll decide if she wants to do it or not. You can force it on Clementine, but you're a big arse if you do that. I don't think I've seen a play through where someone does it. The idea makes me personally feel really horrible, but it's there, if you really wanna poke at Clementine. I don't know how many people actually did it, and even our QA guys were pretty surprised that it happened.
Beyond the coolness of that being a timed decision that might be out of your hands if you wait too long, oh man! I can't get over the fact that I did make her kill me and I may, in fact, be an arse. Is it really so rare to force Clementine to kill you? What did you guys choose?
I just kind of figured it would be worse for her to go on with her life knowing that I'm not actually 'dead.' Moreover, knowing how stuff works in the world of The Walking Dead, Clementine would have to make decisions like that — and much worse — in the future. If the point of that final interaction is to teach her stuff, that's a valuable lesson, no? You'll have to kill the ones you love.
...listen to a giant jerk try to defend her decisions. Hah. I take some solace in the idea that there are no good guys in the world of The Walking Dead.
Faces of Death, Part 5: No Time Left [Giant Bomb]