How Long Before Filter Wool Needs Changing?

Hi everyone, I have a fluval u3 filter which came with my tank, it's been running for 2 months with fish. When I first started using it, so about 3 months ago (fishless cycle) I removed the carbon off the cartridges and replaced it with more filter wool (this filter has 2 compartments with a sponge and a carbon/filter wool cartridge and 1 compartment with ceramic media). As per advice on here I used the remaining part of the cartridge to secure the filter wool into the compartment against the sponge if that makes sense.
My question is how long until I replace this additional wool with fresh? I rinse the sponges and wool in old tank water about once a fortnight when they get a bit brown. My water is clear and parameters are normal, but I know you're not supposed to change it all at once so I don't want to wait for a problem to crop up before I need to change it, what do other people do?
The cartridges which I removed the carbon from are looking a bit thin too.
Thanks

I find that filter wool does not wash well - it goes thin and holey. When I use it, I need to change it every couple of weeks. If it is changed this often it never gets chance to grow many bacteria, and it is not a particularly good home for the bacteria anyway as it doesn't grow biofilm properly.
If you buy it by the metre and cut it up to shape it doesn't work out very expensive even changing every 2 to 3 weeks.

I have a U3.  The first thing I did was throw the carbon and the filter wool out as it's not needed really.  Filter wool is used as an extra layer to polish water (catching smaller detritus than the standard foams) and not for bacteria growing.  It doesn't last long and is more trouble than it's worth IMO.  I get large pieces of black bio filter foam off amazon for cheap and cut them down to fit filters.  Saves tons of money against buying the brand specific bits and bobs for each different filter I have.

Does the black biofilter foam keep the water as clear as the wool? I might order some.