It's not long, you should be able to complete it in under 10 hours easy, probably 6, but you have challenges if you like those to work on, some are really hard.
That doesn't sound too long, since I'm not a huge fan of going for objectives.
What makes the game so awesome for you, by the way? Just asking because I've heard pretty mixed opinions of it.
Just the catharsis of playing it, it's gorgeous. I cannot believe they don't have a PC demo (There was an XBLA one). It is just amazing, and the first time you see a tsunami you will crap yourself. If you liked Populous you will like it, trust me it's a little different and not "hard" in the traditional sense (though you will certainly have to redo some levels), it just rocks.
And I'm on painkillers so I'm having trouble communicating, sorry. For that price, trust me, you're getting the game play out of it.