In New England I would wash / scrub it 1x a month in general May-Nov.
I would "seal" it yearly with something like
this protectant or I've tried 303 Fabric Guard. I felt the Porsche branded product actually gave better water repelling properties than the 303 stuff. (Wife and I both have had several convertibles over the years). I tried the Porsche branded product on recommendation of the local German car repair shop that worked on our Audi's / 911S.