i change mine out every 7 days,religiously.when they are really sucking up the nutes,i have changed out at 5 days.in the early stages of growth,i have let them go for 9-10 days.they are not using much of the nutes then.but when they hit the stretch,they get hungry as hell.a plant i have now in flower was sucking up a gallon a day for a week or so.then she tapered off to about a 1/2 gal. a day.