When it needs it
Some manufacturers say their bottom paint will last as long as 3 years, but that's like saying a battery lasts 60 months; it depends upon the conditions, I think. Now, I remember that stuff they used to slap on the bottom of boats near the shipyard where I lived as a kid growing up in South Mississippi; that junk, whatever it was, never seemed to wear off of anything...including you if you got some on you!But, times have changed a bit and I just start with a high-quality bottom paint -- this year it was Interlux Ultra -- and I expect to get at least 15 months out of it; maybe more, but I expect only 15 months. I keep an eye on the bottom and recoat it when it looks like it needs it. In the long run, I guess, bottom paint can be cheap compared to the alternatives.