Answer:

It depends on a few factors. 1) are you closing your pool every year and therefore draining and refilling with fresh water at least a bit. 2) water chemistry - the more you have to add, the more your total dissolved solids will increase leading to the need to drain and refill. 3) evaporation rate - again as water evaporates off and you add fresh water you are keeping the water fresh and diluting the TDS. You should be able to go at least 6-8 years in between complete drain and refills.