I can easily beat BK 100% in 4 hours now, but for what it's worth it took me 60 hours the first time.
It depends on how you play. It'll last you a lot longer if you're not one of those types who have to 100% each level before moving on to the next.
It's one of the best games ever regardless so play it.
And for the love of all that is sacred don't play the dumbed down XBLA version.