Out of all my 3DS games I've played Ridge Racer the most (Street Fighter second). I've done about 60 per cent of the career mode and I've clocked about 12.5 hours. The races are starting to get more difficult now, so the last 40 per cent my take some time.
Overall it's well worth picking up if you want a good arcade racing game, but be aware it is very similar to the PSP Ridge Racer games, so if you have those then you may want to rethink buying the 3D version.