Depending on whether the river is going up or going down and how fast the flow has been changing, I generally find anything below about 6,000 cfs to be pretty fishable in a DB.

Much higher than that and you have to start looking at fishing secondary water as the main holes are probably pumping too fast.

Low flows are nice because it keeps the jet boats off.

That being said, how it got to the current flow is more important than the actual flow. It can be blown with 6 inches of vis at 3,500cfs and a beautiful green and fishing great at 6,500cfs.