I'm doing every side quest, plus tons of race stuff and trading/upgrading/racing and just some average amount of exploration... and I have only nicked the first 1/4 of the game, if not more. I could easily see this being 20 hours+ if you did everything the game has to offer, and had a few extra races and whatnot for cash or just for fun (which i <3 the racing elements here and offroading... its just epic)
game just keeps getting better and better as it goes along, TONS of content and stuff to do. Not quite as much exploration and bartering as fallout, but offers a entirely different experience that Fallout, and by far well integrated into the action.
