As you can expect from an episodic game, it's not very long. About 3-3,5 hours. I know, it seems short at first, but the game has good replay value. There are some randomly generated puzzles for example. And the game itself is very very fun to play and this alone makes the game replayable.
I'd buy the complete season. From the Telltales' site:
Buying the entire season gets you access to each episode as soon as it comes out on Telltale's site (we'll send you an email to let you know it's available). Next spring, you can get a CD version of Season 1 for the price of shipping. (You don't pay for shipping until then.) Remember, there will be six episodes in Season 1, which means buying the whole season up front will save you almost $20 over buying them individually for $8.95 a piece.