1. How much will I actually be able to jump if I exercise regularly?
This depends on whether you just want to stick to the exercises I described or get yourself a copy of Jumping Manual. In the first case, you will get about 5-6 more inches to your present leap, whereas following the guide will help you get at least 10-13 inches in about 12 weeks. If more time is spent training, you will probably develop a 40+ inches vertical leap.
2. How many days a week should I exercise?
Well, 7 would be the right answer, but you can even go with 5 days a week. The important thing is to keep it up.
3. And how many hours a day?
This actually depends on what stage you are are with your training. Beginners usually exercise about 30-60 minutes a day, depending on their commitment.
4. So all I have to do is exercise and you can guarantee I'll jump 20 inches higher?
Basically, yes. But in order to improve your vertical leap faster, you can take some supplements that will allow your muscles to develop more quickly. More useful details can be found in Jumping Manual.
5. Is basketball all these exercises are good for?
Not at all. Volleyball players, handball players, or even swimmers can use powerful legs, too.