I know there's no better term (except "episodic game series," which is a mouthful), but Telltale really shouldn't call their games "seasons." It makes people associate them with TV seasons, which in turn makes them think of a regular schedule.
Anyway, OP, the only past Telltale game to get multiple seasons is Sam and Max. Season 2 began seven months after Season 1 ended. (This was back when Telltale had a lot less on their plate.) And Season 3 began two years
after Season 2 ended. So there's no real way of guessing.
What we do know is that Telltale's next game will be based on the comic book Fables,
followed by a revival of the classic King's Quest series.
Odds are we won't see Season 2 of The Walking Dead until these games are done, so my guess is sometime in 2014, at the earliest.
Then again, with how popular this game's been, maybe Telltale will bump Season 2 higher on their priority list. I hope they haven't forgotten King's Quest, though.