How many times should I jump on my mini trampoline a day so that I can build up my legs?

You'd be better served with other resistance training...

You need to strengthen your legs and your core explosiveness.

Toe raises, squats, lunges, ab and back work, and "explosive" motions with weights. Not to say you won't gain from a trampoline, but id focus on a combination of weighted lifts that hit your legs, back and abs while focusing on strong, fast, explosive motion.

I use my rebounder about a half an hour a day. I have a bar to hold on to for balance that also lets me do much more of a variety of moves for my legs. Your legs will definitely be strengthened by using the rebounder on a regular basis. I find it gives me a great workout and gives me a great cardio workout as well.

I have bad knees and if I did all the exercises cited above I'd be in bed unable to walk for a week. The rebounder is easy on my knees.