1) In most average household applications (i.e. two standard-sized 10″ filter housings mounted together before a UV system) — rainwater and well service — you should be changing your filters out every 2 -6 months. (2-6 times per year). If you’re changing more than that, it usually indicates a problem with well turbidity, pumping zone; and with rainwater systems, it means your cistern is overdue for a cleaning and your collection pre-filtration (off the roof) is in poor condition.
2) Good polypropylene depth filters should trap through the entire depth of the filter. So don’t be alarmed if it looks filthy on the outside in a week or so. Shine a light up through the bottom, and if you see white, it still has capacity. The other sign to watch for is flowrate or pressure at the tap. If it starts out strong then the pressure seems to taper off after a few seconds, it means the filters are loaded and overdue for changing.
3) When changing your filters, always shut-off water at valve before the filter, and turn a downstream tap on or otherwise relieve water pressure on the system BEFORE you try to remove the filter housing. Otherwise you will struggle to remove the housing and eventually stretch your filter housing O-ring out of shape and it will cause leaks. Use your filter wrench to remove, keep your O-ring clean, wet and grit free, and firmly hand-tighten to put back on.