A simple wax twice a year should suffice. You could do it 4 times a year if you wanted or even 52, it only takes maybe 10 minutes to do both.
I wax the headlights whenever I wax the truck, so once every 3 months.
I am going to use Flitz. It has been around a long time and in the marine industry, it works great for polishing everything from aluminum to lexan and isinglass. It does not have abrasives and will bring things to "new" condition with the buffer attachment on a drill. My lights are fine, but the wife's Altima are a mess. I used the 3M kit on them a few months back and it worked OK but wasn't very impressed with it. Will let you all know how it goes after this weekend.