I would say that any amount that does not interfere with your daily activities is a normal amount. If you’re skipping opportunities to hang out with friends or to go to events in order to stay home and masturbate, then you’re doing it excessively and you need to address it.

Or if you have an actual sex life and your sex life is suffering because you masturbate so much that you don’t want to have sex, then I could see that being a problem.

But, just like drinking, if everything is going well, and if you’re keeping it under control and you’re not damaging your interpersonal relationships with it, and you’re keeping your wits about you to a certain degree, then you’re perfectly fine to do it as much as feels reasonable to you and your own body’s drives.