Ideally though an average would be kept of uses. For example, knowing that the filter needs to be replaced after 640 refills from our municipal water source the average uses can then be averaged out to know the timeframe that it should be changed out. Our family takes water bottles with us on outings about 3x a week and we usually refill it once per outing so our filters need to be replaced every 106 months or about every two years which can be marked on a calendar/planner ahead of time. If we were to begin using them far more often or refilling from non municipal water sources it would of course bump up the timeframe.
Hope that is somewhat helpful!
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