Since I did not have the opportunity but a pro shop visit, it took me 1 1/2 years to get a polish on my oldest ball... personally, I'd suggest to polish a ball when you can see a track and/or the polish is simply worn down. Noticeable loss of recation is also an indicator, although that's not easy because ball reaction changes only gradually over time.
I would not do it too often - it is like cooking a steak. Too much, and it is done.
Additionally, there is the option to sand the ball beforehand to get rid of scratches and the uppermost coverstock layer. If you treat the ball well (keep it clean etc.), this should not be necessary more often than once a year. Maybe even less. A polish to get a high gloss ball back to box condition could be done more often, but I do not think that it is needed more than 2-3 times a year, even with heavy ball use.
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DizzyFugu --- Reporting from Germany
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