The filter is the decisive factor. Of course, you don't have to count how many litres you have drunk, after the approximate 4000 litres have expired, it is blocked and you can't suck out any more water. Then you have to change the filter. Activated carbon filters can be changed after 1000 litres, but this is only a flavour enhancer of the water.

Filter after 4000 litres or when nothing more comes through and active carbon after 100 litres.