If using proper washing technique then, as other have already stated, once a year or every other year. Those quick wiped owns with DS here and there and even with the best wash technique you are still touching the paint so there is opportunity to induce some swirls. My last vette went 3 years between polishing once., and it didn't really need it, i just wanted to do it lol
So once you have the paint where you want it just do your normal washing and waxing and you should be good for a long while.
Unless it's black then the job is never finished lol
Anold ride, gone but not forgotten