I empty my collection cups & clean them 2x per week. I tend to have fairly heavy bio-loads in my tanks, so the skimmers all work overtime. I find that if I go much longer that that, the goo builds up in the neck of the skimmer/cup, and teh effectiveness is greatly reduced.
For the record, my 90 has a Remora Pro with pre-filter/surface skimmer box, with a Mag 5 pump, my 40 has a Kent Marine Nautilus TE in a 10 gallon sump, and Mag 7 pump, my 20 gallon QT has a CPR Bak-Pak2 (with bio-bale) running on a Maxi-Jet 1200, and I have another Remora Pro with Rio 1400 doing duty on my live rock curing barrel.