Grayl Ultralight Replacement Purifier Cartridge

Backcountry.com says:
Replacement purifier cartridges for the UltraPress® (and Ultralight) are designed to last 300 cycles (40 gal. / 150L). Lifespan will decline when purifying water with high levels of silt, sediment or tannins. Cartridge press time increases towards the end of the cartridge lifespan; when press time reaches 25-30 seconds, the cartridge should be replaced. Cartridge continues to protect, even as press time increases.