Masturbation does not cause any kind of weakness. Many people relate the pleasurable relaxation feeling during orgasm with weakness. This is a myth.
The pleasurable relaxation feeling is caused by the release of nuro chemicals called Endorphins. Endorphins in moderation is good for the body. You can masturbate as often as few times a week. It's hard to say how often is normal as it varies from individual to individual but as long as you are not hurting yourself, it should be fine.