Even if you don't play Fable 3 more than once, you can expect about 20-25 hours to play through the core game, and anywhere from another 15-20 more if you decide that you want to see most of everything the game has to offer. But I'd recommend not charging straight through – go wander about and sniff the proverbial flowers and have fun exploring the world of Albion.
And watch out for the Killer Poultry. It'll knock your socks off. Or at least set them on fire.