Changing optional KH inline filter- do you & how often?

Replacement filters are not cheap if you have to replace every month as I once heard. I draw sample water from a small dedicated frag tank running from my sump as this close to the KH director. it has lighting and gradually encrusts with coraline algae. This must gradually impede the water intake. How often is it advised to change the filter and can any mileage be gained by cleaning it in say citric acid solution?- I could take water from the sump but that would mean adding 3m to my intake tube length which may impede accuracy?
What do others do?

How long the filter lasts will ultimately depend on placement and how many tests per day the KHD is doing.
Keeping it exposed to light is not the ideal placement since it will be exposed to algae growth as you've already noted. If it's only the outside getting covered, that is fine as long as it does not impede on the water sample itself. A general indicator is when the filter becomes visually dirty or at most, when the KH test results are "off" from their normal values.