Running mine I'd always flush the first day between veg and flower and then one day between week 4 and 5. You're not running a standard grow medium so there's no "build-up" to concern yourself with flushing. Your reservoir is literally giving you "run-off" readings every time you check it which allows you to change your nutrient solution immediately on the fly instead of having to flush out improper feed issues out of media such as soil/coco