How long would the average cartridge filter last ?? I just noticed my heater "LO" light goes on now at 2000 rpms. I tested the flow when new and I could go as low as 1700 rpms with my VS pump. Just for safety sake I increased the rpms to 2000. The pool only has been opened for two weeks now and I'm thinking the filters are dirty. The gauge on the filter doesn't indicate high pressure either. Just curious. Thx