Depends on the surface of the ball, and whether you're talking about a touch up, a refacing, or a complete re-surface.
A touch-up entails throwing the ball on a spinner and hitting it with an abralon pad or some polish. A refacing is getting as close as possible to replicate the factory finish (i.e. 800, 800, 1000). A complete re-surface entails sanding the ball down to get rid of scratches, gouges, and the such and then bringing it back to factory condition.
Since it sound like you're talking about a touch-up, in my experience a polished ball doesn't have to be touched up as often as a matte surface ball. I touch up a ball like the Invasion about once every 6 or 9 games. Whereas I touch up a polished ball once every 12 to 20 games depending on how it looks or is reacting.