Not sure if this is helpful, but it may be good for reference. I would think flow rate would be less important than heat dissipation from the fluid (to a point).
The CW3000 cooler (not a chiller) that I have states 10L/min. It runs around 22°C under constant use with the 40W laser. It seems to do well enough for me.
Keep in mind, the cooler has two aluminum coils with a fan to force air over it to dissipate heat. I'm just using distilled water (no coolant).