We are full-timers.
Camco doesn't list a gallon life for the KDF 'Blue' filter sold in many places. The company web site says 'Large Capacity Filtration: Large capacity in line water filter lasts an average of 3 months.'
We use one as the first step in filtering our water, and I change them on the first day of each quarter. I also change both the charcoal caratridge and the sediment cartridge on our two stage filter the first of each month. About $60 a quarter in costs.
I would say on average day after day, we run about 15 gallons a day through the filters when we are hooked up to sewer, which is about 80% of the time. Guessing between 1,300 and 2,000 gallons per quarter.
What I've been able to research is that is a pitifully small amount of water to run through a filter and think it needs to be changed.
I was more concerned before we went full-time, because the water filter would sit wet for weeks between trips. Supposedly there will not be anything grow inside the filter, but it is a perfect environment for that to happen.
And we have experienced some water systems that make the filter useless after only a couple weeks.
It's an experience thing. After a while, you should get a feel for changing it. One thing I strongly recommend is buy TWO. Always have a spare, and if the water tastes or smells in the slightest 'funny' go change it right then. Then flush the lines is you have a sewer connection or the gray tank capacity.