Cartridge elements need to be routinely cleaned (weekly) to ensure optimal performance. Every month you should degrease your filter with 'Renew'.
Your element will need to be replaced when it has started to wear. This can be seen by 'fraying' of the pleats, or gaps opening between them. This sinifigantly restricts the filters ability to maintain spa water clarity, and ultimately resulting in poor water flow and controller errors. In turn this will drastically increase your spa's running/chemical cost.