Every spring, if you don't mind brushing. That's an aspect of old wooden boats I've learned. Once a fiberglass boat like mine gets so old, you can forget gelcoat. It's history!

So if you don't mind spending a few hours with a paint brush, your boat can look new, every spring, like this one built in 1959. They paint the topsides of a boat like this with a brush and it takes just a few hours and a couple quarts of enamel. If it's done every spring, prep work is minimal. Of course the cabin gets a coat of varnish,as do spars, if they're wood.

Presto, new boat!