Many under the sink water purification systems have replacement water filter cartridges which need to be changed every 600 to 1200 gallons; unless there is a gallon-o-meter attached to the average persons faucet, the number of gallons used could be somewhat difficult to determine.
One simple way to decide if it is time to use a replacement water filter is if the sink begins to loose water pressure; the sink will loose water pressure because the filter is clogged with dirt or other sediments. Depending on the microns the filter is able to keep out of the water some units may be washed out and reused for a time; usually if a filter stops less than 5 microns from escaping the filter, rinsing out the filter is useless.
A good rule of thumb is that an average household will use about 600 gallons of water per month from their kitchen sink for drinking, cooking, and cleaning; using the replacement water filter before the water actually starts to taste or smell bad is a good idea, however waiting until there are signs that the filter is loosing effectiveness is a good idea to prevent waste.
Whole House Units
Whole house water purification systems are the...