>>No girl is letting you fuck her 50 times a day, that would take a barrel of lube, even for an 18 year old nympho.
You don't have 50 episodes of sex. What happens is a series of orgasms during a single episode of sex, one after another, and you still don't feel depleted afterwards. Usually I go 30 or so minutes in missionary, then my arms/chest tire from holding myself up. During that 30 minutes I have as many orgasms as I want, at least 20 but 50 is possible. I try not to count, but rather focus on enjoying the moment. I finish my current girl with cunnilingus because she can't cum unless the man has a peak orgasm, which I refuse to do, and even then it's not certain with her. Cunnilingus gives me much better control.
There's also sexual kung fu with multiple orgasms for women, and Mantak Chia has books for that, but no girlfriend had ever been willing to listen to my talk on this subject: in one ear, out the other. I have known several multiply orgasmic girlfriends, however. Ability to have multiple orgasms, one after another, is much much easier for women than men. One old girlfriend in particular used to have 30 or so violent screaming orgasms in a row, finally begging me to stop. It wasn't that I'm some great sex God, it's that she had incredibly strong pelvic muscles, which is the key to multiple orgasms. Also very strong core muscles, which is closely related. She didn't realize what a prodigy she was, though she mentioned lots of men told her she was incredible. I guess she thought we were referring to her looks (she was a looker, I must admit) or personality (batshit crazy).
And yes, solo cultivation of enlightenment, or sharpening the pencil as Mantak Chia calls it on one of his videos, but which most guys refer to as masturbation, is an important part of mastering sexual kung fu. Dual cultivation is just way too difficult until you've first mastered solo cultivation. I have injured my penis in the past (rubbed skin raw) from excessive masturbation but I learned my lesson and know when to stop now.