So I'm close to 40 and libido is certainly not what it was when I was 20, but I would still say on average 2x a day. Usually in the morning and before going to sleep. This doesn't seem to change even when I'm having weekly/daily sex w gf. Stimulants (cocaine/amphetamines) certainly make me a lot hornier and more willing to masturbate, but they also don't help in the dick department if overdone.
My one serious methamphetamine bender, my dick stopped working, but simply touching myself was orgasmic.
Current gf is actually really cool. She does not mind at all, and probably encourages this behavior. She's not nearly as sexual as I am, so maybe it takes some pressure off her. An ex gf for contrast, completely freaked out on me when she caught me once. Like it was almost the end of the relationship.