which skimmer? it really depends on ur bioload, and i wet you skim it. I remeber i had tunze 9010 and 9020 in my 120 g b4, but my tank were empty just live rocks. basically when you first start it, try to have about 1" of foam at collection cup, that makes it skim dry, and maybe after few days make sure it's all stable, you can run it more wet ( maybe half " or less ).
i loved the tunzes, very compact size, and i remeber when i skim it dry, i clean it about every 4 days, and wet, i clean it every 2 days. The only part i don't like about tunze is that it's alil bit hard to do minor adjustment with the lil screw.