The Sta-Rite SYSTEM:3® Modular Media Filters - SM Series has the industries longest lasting cartridges of any Cartridge Filter. I usually get 8-10 years of use out of them before I replace them for my customers. As long as you clean them at least every six months they should be very long lasting. It is a good thing too that they last so long because the replacement cartridges are very expensive. I buy only the Sta-Rite OEM cartridges when I replace them.
You can see from the video that the 450 sq ft ones when compared to a 420 sq ft Cartridge dwarfs it and just by the overall design make it a superior filter over all other brands on the market. If you are looking to install a cartridge filter and you have the room for it at the pool equipment, I suggest going with a Sta-Rite System 3 Media Filter (Cartridge Filter). The problem is that these things are so massive that many pool owners don't have the room to install one.
When replacing the Cartridges I suggest replacing both at the same time. It just makes sense to keep them consistent and replace them as a set. The filter tank and filter parts are extremely long lasting so you will just need to replace the cartridges when they were out as the filter tank itself will last for decades. It is that well made.
Be careful when removing the old cartridges, especially the large one. If it is wet and covered in dirt it can be quite heavy. You can let it dry before removing it or get someone to help you. If your just plain tired of these cartridges you can also try the Retrofit kit for the 300 sq ft and 450 sq ft filters. These are easy to clean but won't last quite as long as the OEM version.
Here are some for sale – note that I do not know this particular online pool store but it seems okay:
Here are the four different Medias and links to purchase. The most popular sizes are the 300 sq ft and the 450 sq ft so chances are one of those is installed. Check the label on the filter to confirm your Media size.
S7M120 300 SQ FT:
Sta-Rite 25021-0200S & 25022-0201S System 3 S7M120 Swimming Pool Filter Set - https://www.amazon.com/Sta-Rite-25021-0200S-25022-0201S-System-Swimming/dp/B00768TP7I/ref=as_sl_pc_tf_til?tag=swimmingpooll-20&linkCode=w00&linkId=de3d6b3de216ac61e2195b3f8318cced&creativeASIN=B00768TP7I
S7M400 400 SQ FT:
Sta-Rite S7M400-SET System 3 S7M400 Modular Media 400 - Small and Large Cartridge Replacement Filter Cartridge Kit -
S7M450 450 SQ FT:
Sta-Rite System 3 25022-0203S+25021-0202S Swimming Pool Filters Set S8M150 - https://www.amazon.com/Sta-Rite-25022-0203S-25021-0202S-Swimming-Filters/dp/B00JVS3PQS/ref=as_sl_pc_tf_til?tag=swimmingpooll-20&linkCode=w00&linkId=c2fc42ed66a8d0d28f4d087fb08eb542&creativeASIN=B00JVS3PQS
S7M500 500 SQ FT:
Sta-Rite 25021-0224S + 25022-0225S Full System 3 Pool Replacement Filter S8M500 - https://www.amazon.com/Sta-Rite-25021-0224S-25022-0225S-System-Replacement/dp/B00LPHW848/ref=as_sl_pc_tf_til?tag=swimmingpooll-20&linkCode=w00&linkId=271d3f279e5c70fa1f96e3a2d4644880&creativeASIN=B00LPHW848
STA-Rite System 3 Filter cleaning: http://youtu.be/d3q19HIm12w
Sta-Rite System 3® Retro-Fit Kit Review and Installation Video: http://youtu.be/ylcO7jTZ9pE
Sta-Rite System 3® Retro-Fit Kit Part 2 of 2: Cleaning the Cartridges: http://youtu.be/waSXKoHZfnwInstalling a Pentair 98209800 High Flow Relief Valve on a Sta-Rite System 3 Filter: https://youtu.be/e3eS2WNawZ8