How clean do you want to spend? There are some anti-fouling coatings for props that seem to work well (at least that's what the proponents say), and they are quite expensive. Other folks use regular anti-fouling bottom paint with mixed results. Some sailors swear by egg whites. Yes, egg whites....applied to a shiny, clean prop. About three coats. If I ever try it myself, I'll report back. How often you have to scrape your prop to remove growth depends on how you use your boat, where you are, how effective your annodic protection is, how 'nutrient rich' the water is, and what anti-fouling coating you applied to it, etc, etc. Inspect the prop regularly, because it's much easier to scrape off a little vegetation or a few small barnacles than try to clean a prop that is so encrusted it looks like a volleyball.
When I've left our boat in the water for more than a month without moving, I've put a heavy, dark plastic bag around the prop. Tie it tightly to reduce the amount of water that circulates in/out. Barnacles don't seem to like stagnant water and darkness. Remember to remove it before getting u/w.