I was always told that masturbation can do no harm, and can even help maintain your sex drive.
Once a day is certainally not excessive, and I would guess that is what I average even now at the age of 30 (although like you I do give myself the odd days rest too

)
And yes, I am married, so I have sex a few times a week also.
This habbit has gone on for 15 years now, and I don't think it has harmed me. My wife has done the same since she was 15 too, and we often just masturbate in front of each other as an alternative to sex. (which is GREAT fun !!)
Just enjoy it !!