Several plays. I like Prosperity a lot as it made the game feel fresh. It changes the game a fair bit when played with the Platinum and Colony cards. Have only played Prosperity with a smattering of base cards so far.
This expansion makes play more gamer oriented than the original alone. However, it does increase game time noticeably, but it's still about 1 hour play which is excellent. I'm just as happy to play vanilla or with this expansion.