ive seen these answers range from
1/3 per min (yes a THIRD)
to
5 L's per min
persoanlly mine is around 1 L, i dont think you need anymore than that, so why waste water ?? ?? ??
or course i am sure the guys with the 1000 L systems will say "thats why i do a better job, cause i use 5 L a min, get a van system like us big boys"
but ive been at it 1 year now, and 1 L a min does me well enough