Your eyes should tell you when. Look at the track area. If it has dull looking lines after you have thoroughly cleaned it, then the polish has worn off by contact with the lane. Refresh it.
In general, I'd suggest at least every 18 - 25 games. If the lanes are on the dry side, if you're rev rate is on the higher side, the lane surfaces are older, then the number of games a polishing will last will be less to much less.