I was wondering if overuse of masterbation, over use in my mind being anywhere between 2-4 times a day, can temporily or permantly cause penile numbness and if temporarily so, would one have to abstain from masterbation for awhile to have sensation return and how long would one have to abstain from?
I would think having a sore penis would be a bigger problem than any potential numbness. Masturbating a lot will cause the nerves on your penis to grow temporarily numb though this should go away on it's own within at most an hour or two. If you feel a distinct numbness after having masturbated several times in a limited time period I would suggest that you let your friend rest overnight or at most a day or two. I've never heard of the numbness becoming permanent and I would imagine that you would have to abuse yourself pretty badly for such a thing to occur.
Just keep it in moderation like everything else and you shouldn't suffer any ill effects.
Though if you still feel numb several days after a more excessive session I would strongly urge you to seek consultation with a licensed urologist.