Every six months. I just did. If you pay monthly you're either poor or can't do math. (Or your company doesn't offer good paid in full deals)
As an insurance agent for 35 years, you're a bit too narrow here in your thinking, Red.
Even with a huge paid in full discount that companies like Progressive (we represent them, I know) have, every company rates different factors with different weight. My company insures my two cars and home together and I pay monthly on both policies. I get a huge multi policy discount which offesets Progressive's paid in full. My company does not charge service fees for the automatic pay plan either.
People like yourself with one vehicle and maybe a renter's policy might very well get a deal using a paid in full program, even some with conditions like mine it's worth it, but it's not always the case.