It varies depending on what part of the lane I've been playing. But to sum it up, just when the polished has been "burned off" to where it's gone. This can range anywhere from 20-30 games?
#1, I would NEVER apply polish by hand, or #2, as often as you're describing with the father and son. They have probably completely clogged the pores of the ball, thus making it seem like it has died because there's no friction to allow it to hook. Just because the bottle says you CAN apply it by hand, it doesn't mean that the results will be the same as if it were done on a spinner.
I do, however, clean the ball after every use with ball cleaner or a Simple Green/water mixture, but that's entirely different from adding polish each time. Hope this helps.
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Lane Carter, Strike Zone Pro Shops - Salt Lake City, Utah