i change mine day 13 or even 14-15. i actually see more growth and ph stability after day 6-7. i top off by mixing nutes/ro in 1gl jug and add, if ppm to high, just add ro.Are you saying some people never change the reservoir water?
Sounds like I've been doing something in between. I change my reservoir every 7-10 days with fresh nutrient solution and Hydro guard. I've been topping off with freshwater pH water as needed.