Probably once a year, or when pressure rises.

Every 1–2 years, but check the pressure gauge first. That’s the real answer.

Most cartridges last a season or two, depending on how much junk gets in your pool. The gauge tells you everything: when it reads 8–10 psi above the clean baseline, it’s time to replace. Also pull them out and look for frayed edges or holes—if you see daylight through the fabric, that’s a hard no.

A well-maintained cartridge (rinsed monthly, deep-cleaned twice a season) can stretch to two years. But cheap cartridges might crap out in one. Don’t wait until the flow is a trickle or the pump starts straining.

Cheaper to replace on schedule than to burn out your pump.