Bit confused here Alex, I normally use the old type filters and change them every six months. I use about 2.5k pure water a week. Have I left them in too long? Would I benefit at all using the new type? The info says they are good for about 38000 litres.
Most window cleaners (myself included) do not change the filters soon enough. Every carbon filter has a recommended chlorine reduction lifespan in litres. Technically once this amount of untreated water passes through the filter its protection of the RO membrane is reduced.
As a supplier we have to recommend changes inline with manufacturers lifespans as this will properly protect and extend the life of the membrane.
With the Fibredyne filters they have a lifespan of about 38,000 litres which means that when this amount passes through it (not the amount of product) it is time to change. The ordinary budget filters that most people use will have anywhere from 5,000 - 12,000 litre lifespan (depending on the supplier). In reality this will mean that you would need to change them every 1-4 weeks for maximum RO protection based on your water usage.
Whatever you get from your standard triple-pack you should be able to get from a single Fibredyne filter and then some again. However after 6 months of being in water there is a good chance that any filter will have exceeded its useful life, regardless of quantity of water through it, and will actually start to hinder production (including your current triple packs).