How often do I need to buy new cartridges for my cartridge filter?

Like with answering how long should my filter cycles should be, this answer is dependent upon how the pool is maintained. Cartridges in a cartridge filter are paper and if they are taken care of (cleaned as needed, chemically soaked occasionally) they should give you years of service. On a new pool, the first year of usage is likely the most difficult for the filter as it will be filtering out the largest amount of debris (landscape debris, more dust than usual, etc.). Many customers have a spare set on hand but cartridges should last for several years.