As often as I feel the need for sexual release and there isn't a better method (sexual activity with a partner) available.
If you're hungry, eat. If you're thirsty, drink. If you're bored, be entertained. If you feel sexuality frustrated, fornicate (with a consenting adult) or masturbate.
If you're hungry, eat. If you're thirsty, drink. If you're bored, be entertained. If you feel sexuality frustrated, fornicate (with a consenting adult) or masturbate.