A lot depends on what kind of water you are in. I got tired of seeing the significant loss of effectiveness of multi-season paints after the first year. When I upgraded to this boat I threw in the towel and went with an inexpensive single-season ablative. It's $90/gallon, and I need about 1-1/3 gallons per season. So I just put on one thin coat every year. I haul out every winter, so it's part of the springtime prep.
https://www.go2marine.com/international-paint-proguard-ablative-antifouling-paint-gallon-blue-black-red
I used to sand the whole bottom diligently every year, but the last couple years I just do a hard scrub along the waterline to get rid of any algae stain, let it dry, and put on the paint. (It helps to have a good power-wash at haulout.) It's a half-day of work. I see no signs of paint failure anywhere on the bottom.
https://www.go2marine.com/international-paint-proguard-ablative-antifouling-paint-gallon-blue-black-red
I used to sand the whole bottom diligently every year, but the last couple years I just do a hard scrub along the waterline to get rid of any algae stain, let it dry, and put on the paint. (It helps to have a good power-wash at haulout.) It's a half-day of work. I see no signs of paint failure anywhere on the bottom.