Endeavor - I think it depends on the quality of your barrel. I have the cheap T/C Omega Z5, and if I don't clean after shooting BH209, I have rust (high humidity or not).

every time I clean my rifle, which is every time it has been shot. some people like to shoot with a fouled barrel, but I have always sighted in with a clean barrel for every shot. That way I always have a clean bore and POI stays consistent.