The life of your pre-filter cartridge will depend largely on the source water being used. Drawing water out of the open ocean may allow your prefilter cartridge last for several months, while drawing from polluted bays may foul your cartridge within hours of use. The cartridge is a 10″ x 2.5″ 5 micron pleated poly filter.
These inexpensive cartridges are widely used and available throughout the world from water filtration retailers and hardware stores.
Do not use a 5 micron carbon filter as this prefilter. It does not provide sufficient flow to your system and will cause performance issues by starving your high pressure pump of seawater.