I recently left my RIB in a marina for 6 weeks, with use about every two or three days.
I did not antifoul as once its on there is no turning back.
After lifting the transon (where their is most light) was very weeded, the tubes and bottom of hull had little weed but lots of slime and a few barnicales and other hard things, not very dense though.
It took about 6 hours with a powerwash and scrubbing to remove the fouling (see post previous) but in hind site it may have taken somtime to put the anifouling on anyway.
My boat will be doing the same next year and in my case i wont be antifouling next year.
Things to consider
Is this a one off or will it be every year.
Look at the post powerwashing hyperlon, people talked about using baby lotion which is supposed works.
What time of year is it. I belive that barnicals releae their young about May and your much more likly to get lots of growth then.
How does your boat sit in the water, if the tubes are below the surface you will have to clean them of anyway (or does anyone know a antifouling for tubes).
Although it took 6 hours my RIB came up as clean as it went in, just lots of hard work.
I think its way up the odds and see waht works best for you. If you try it without antifouling and it does not work you can always apply it later, but you cant take it off.