How many hours of gameplay will it take me to complete Rayman Raving Rabbids?

Question

And will it still be fun afterwards?

Answer 1

If you're really determined and very skilled, you can play through RRR in about 2 to 3 days. After that you can improve your score or play the minigames in multiplayer mode.

Basically, the game will only work as a multiplayer game afterwards, as you have seen all minigames, and I personally think the motivation to improve ones score after you unlocked everything is fading very quick.
However when you have a few friends to play the shooting games together, its fun fun fun!