It took me 20 hours to complete the 8 normal worlds, and then the 8 special worlds (no harder in my opinion but excellent just the same), as well as getting three gold coins and gold flagging every level. I am currently on special world 4 with Luigi although its a bit of a kop out as you can't collect the gold coins a second time with him. All thats left after Luigi is the last "super" level that you unlock.
I must say though that the game is tremendous, by far the best I have played on the 3ds. Its the first game I have genuinely not wanted to put down and this is just the appetiser to Mario Kart 7!