you should by a TDS meter....and check your water....when you get high levels over 30 TDS you need to recharge....the resins will exhaust according to mineral content of the wate used..the higher the minerals the shorter life span before recharge....
the TDS meter shows Total Disolved Solids in your product water...the higher it is the more minerals in it...the Ideal range is 0-3 ...but its hard getting it with some filters...I use a 2 resin system with 2 tanks.....and it will get it to 0 but after a few uses it will climb up as it gathers minerals....mine was designed for salt water tanks and came from Germany....it does make Pure water....
AL