Is your sump lit up or not? If you've got all kinds of algae growth coating the inside of your skimmer than cleaning it once every 6 months or so would probably be a good idea. If your sump is kept dark, then a skimmer can go for years without a cleaning IMO. I think I went 6 or 7 years without ever cleaning my old Euro-Reef skimmer and never had any problems. It was actually kind of cool seeing how the chamber took on it's own little ecosystem over time.