Hmm ... I'd say about 30 hours of total game time, spread over several weeks. Of that, I'd say about 20 hours of that is time spent on the storyline. If you were to do absolutely everything there was to do in the game, you could probably push 50 hours.

And yes, there's quite a bit of stuff to do on the side, such as bountyhunting, hunting animals, harvesting rare plants, gun duelling, robbing stage coaches, horse racing, playing blackjack/poker, armwrestling, dice games and so on. There's also a fame/honour system in place, which changes how people react to you depending on the choices you make during the game. Rob and shoot innocent people, and you will become infamous. Protect the innocent and you become honoured. It's fun to hear how people say different stuff to you as you progress through the game.