OK, Mr. Newbie Mylor fella, hi.
I used to keep a not antifouled boat in Port Pendennis marina. 6 weeks was the max that I could leave her in without presenting myself with work. After 6 weeks, I'd haul out and the soft slimey green stuff would easily jetwash off. If left in for more than 6 weeks, barnacles would begin to form. These would come off, but with more effort.
There's a coin operated jetwash at the top of the slip in Mylor, so a monthly spruce-up should be easy for you.
As for wrecking the tubes/gelcoat for leaving her in, you won't.
UV protection is probably as, or possibly more important when leaving a rib on the water for extended periods.
Also, ribs and pontoons don't fit together too well. Make sure that she's well made-up and well fendered should an Easterly get in there. I tend to tie a couple of fenders to the pontoon, horizontally. That way they can't pop out should it get fresh.