I usually let my first res go the first 2 weeks of flower, unless pH is creeping towards that of my source water, indicating the whole res has been 'drank' and replenished. I dont really alter pH between res changes. I set low and allow to drift from 5-7pH. I know some growers who only change out the res either based upon the add-back of water being equal to the system volume or when the res pH reaches the pH level of the source water, which would be indicative of the fact the system volume has been replenished. For me, I just do it every weekend.. :)
Peace
BL