Most folks wax their string about as often as they change their oil... and thats why people constantly need new bowstrings.

There is really no set "time" or set # of shots, its is really an as needed type deal. Anytime your string doesn't feel like it has wax on it.... then it needs it.

If you are shooting in the spring, summer or early fall... then you need it even more. Heat will suck the wax and life right out of a string. I find myself waxing the string after every single shooting session in the summer time.