There is no normal frequency of masturbation, you can masturbate whenever you like. The worst that can happen is that your penis may become a bit sore or over-sensitive (rub some other part of your body in the same way and you will find the same thing). In any group of adolescents, some will masturbate several times a day, others once a day and some only once or twice a week. There are also a few people who don't do it at all, either because they don't feel the urge or because they've been told it's bad and manage to control the desire to do it.