Masturbation a natural activity. You can’t overdo it. Try masturbating immediately after you have just finished. The sensitivity of the penis and your own thought process will not even give you an erection. So go by what your body says. If you get an erection and feel like it , go ahead and masturbate. When the body has had enough , you will not be able to get an erection. You don’t get a head ache due to masturbation. Actually if you have a head ache it might cure it. Now , the question ...how often should you do it. Well it is like asking how often should I pee. The answer is also the same .....as often as you have the urge and the situation permits it.