If the filter is primarily for taste to get rid of chlorine than when you start tasting chlorine it is time to change it. If you are after more serious contaminants then about every six months.
I have a two or three filter set up for our trailer depending on water quality and the first filter is a ceramic carbon that heps get the big stuff is cleanable and protects the other filter(s). This set up with the final General Ecology Trav-L-Pure Camper Canister goes down to the NBC level and will take out bacteria protozoa and viruses. With the three stage/filter I can take water out of streams and lakes and make it potable.