I often wonder what the overflow rate per minute has to be? In say a 1 litre tray how often does the water have to change in those 2 mins in the wash which is the minimum suggested by Ilford?

I have an upright Nova washer which when full holds about 1 litre and it can be made to flow very rapidly but I often wonder if I am wasting water. Would an overflow of 1/2 litre per min be enough so the water changed completely in 2 mins or could the overflow be much slower?