I do 10% monthly. I've been skimmerless for 2 years. Went from a 55 to a 100 and now my RR 90.
Skimmers are great to start off, but I think the corals benifit more without a skimmer pulling everything out that you put in. IMHO.
I use less plankton because I know it wont get sucked out. Same with the other stuff I use. Except kalk.