No, you can't masturbate too much. But if you are male, some theorize that you can cum too often. All jokes about conserving precious bodily fluids aside, it takes a hell of a lot of energy to replace lost sperm and semen. That energy is better spent doing more important things, like keeping you healthy, getting things done, and having sex.

Mantak Chia and Douglas Abrams expound in detail on the unhealthy association between sexual pleasure and ejaculation in The Multi-orgasmic man. Their argument is that men will be healthier and enjoy sexual activities more if they can learn to enjoy the journey more than the final destination, so to speak. For what it's worth, I read their book. Following through on the ideas and exercises presented therein vastly improved my sex life.

So to summarize, I don't really have anything to say to women who ask this question. For the men, masturbate as often as you like, but masturbate to orgasm at most once or twice a day.