Antifouling bottom paints come in multi-season and single-season paints.
Single-season paints require you to paint every season because the biocide will become ineffective after one season.
Multi-season paints come in hard and ablative versions. Hard paints do not wear down in the water. In high-growth areas, it may be necessary to repaint after one season.
Multi-season ablative paints will wear off over time, usually requiring a minimum of two coats in order to last more than one season.
When the bottom paint wears away or doesn't prevent growth, it's time to clean the bottom and apply another coat.