I have one set of cartridges going on 9 years now.

The only reason you should need to replace your cartridges is because they are torn which is often caused by cleaning or they are so clogged that you cannot clean them anymore which is often caused by bad chemistry. So the key to extending the life of the cartridge is keep the chemistry in balance and avoid algae blooms (I have had one in 9 years), and go with a large filter to minimize cleaning and the chance of damage. Also, a small pump helps to reduce the flow rate through the cartridge filter.