How do I determine when to replace a carbon block water filter?
I find with mine that I notice slower flow and reduced pressure which is usually when I change it. Typically this is in the 6-8 month range, but as you mentioned it really depends on your water quality.

I don't believe bacteria growth is an issue but overtime the filtering ability will diminish along with the pressure and flow.

I think ultimately it is just easiest to replace on a fixed schedule. In the grand scheme of costs, if you change it 1 month earlier then you should, over the lifespan of your fridge you are probably only changing it a handful of extra times.

If your fridge has a built-in ice maker then the reduced pressure might be more of an issue - if you notice small ice cubes then this could be a sign its time to replace the filter.