as in all thing :
Moderation!
ANYTHING can be bad for you if you abuse it!
like if you are masturbating once a month on soft internet porn (OK)
one a week (still OK)
once a day (hum.. NOT OK)
more than a day (NOT OK)
*disclaimer* I don't know the real number and I don't think their are number of those kind.
As a rule, most of your masturbation should be made on phantasm of your own, if you can't imagine a phantasm that make you excited, maybe stop porn for a while.
no i'm not a doctor, but common sense is good too, never forget sex is mental, if it was physical it would be 10 second long...