It depends how much your drive. For a passenger vehicle you would want to check the reservoir about every second or third fuel fill-up and add more water.
Our truckers with 2.5 or 5 gallon reservoirs tell us they go about 800 miles or better before adding more distilled water.
If you live where it gets really cold in the winter (minus 20 degrees F.) You should put a new solution in before winter with Potassium Hydroxide, otherwise use sodium hydroxide. If you are a heavy commuter, you should change the solution out twice a year.
If it is a large commercial engine running constantly you should change out the solution every 3 months.