REFER THE MEMBRANE MANUFACTURERS MANUAL OR THEIR WEB SITE. THE FLOW RATE DIFFERS FOR EACH TYPE OF MEMBRANE. THE CLEANING FLOWRATE AND THE SERVICE FLOW RATE ARE ALMOST SAME. THE FLOWRATE IS FOR EACH PRESSURE VESSEL SINCE, ALL THE MEMBRANES IN THE VESSEL FOLLOW THE SAME FLOW RATE.
You may refer to RO membrane OEM's website such as Dow, Hydranautics, etc. For 8" pressure vessel, typical flowrate is 6 - 9.1m3/h per vessel. Just calculate the number of vessel of first stage then multiply with flowrate will give you the total flowrate. Flushing duration is subject to the rinsing of CIP chemical until clean.
I am assuming that you are cleaning each stage independently at low pressure.
I would target flow to maintain a minimum of 40 gpm crossflow. To put it another way, there should be 40 gpm off of each housing, with no production of permeate.
So, if you have 10 housings, It would be 400 gpm at a pressure high-enough to get the flow you need, but low enough not to produce permeate.