Sorry guys I have been a bowhunhter for 43 years and have used the same piece of leather for nearly twenty years, to rub wax into the fibers and NEVER had a problem. As for how often . . . any time my string feels dry (about every day hunting . . . and every other outing at home . . . or when the striung feels dry.
Dick, I do the same. I don't burnish the string, but I do rub the wax in with the piece of leather. I've never noticed the wax not going into the string or have strings that failed early. I make my own strings and although I replace them frequently, they are always in perfect condition when I replace them. I measure every string when I take it off and so far, the maximum creep has been 1/16" on a 90-100" single cam string -after a year.
I'm sure that rubbing them hard with the leather might cause too much heat, but simply gently rubbing the was in, appears to work fine.