Well put it this way, the general daily guideline is that we as adults exercise for 30 minutes per day.
There are no program's in wii fit, you have to govern it yourself.
But I would suggest 15-20 minutes of cardio exercises (running, hula hoop etc..) and then 15 minutes made up muscle exercises and stretches.
Yoga sessions generally last for an hour with 20 minutes of relaxation, however to be honest you'd be hard pressed to do a full session given the limited number of yoga moves with the board, so again I would only use it as a help towards stretching and for 15 minutes tops.