I finished the pool in May 2010. After the plaster dust settled and the water cleared up I cleaned the filter even though the pressure had really never changed. I think it may be due to the size of the filter relative to the size of the pool. It didn't look bad considering how much plaster I sucked up with the vacuum. It's a 450 sq. ft. cartridge filter and the pool is about 15,000 gallons. Since the beginning of last May (almost 11 months now) the pressure has only gone from 15 psi to 17 psi. I've heard the rule of thumb is to clean the filter after it rises 10 psi. At this rate, that will never happen. Is there a time frame that you should clean it out at if the pressure doesnt change?