My coop is built within a copse of maple trees (for shade and to assist with element protection). I regularly hang fly traps - the type they enter into and drown, that can hold hundreds of flies - throughout the copse and change them several times throughout the spring and summer. I noticed a dramatic decrease in filth flies once they got going, and the traps basically fill to capacity. My coop is a wood chip based DLM - in the coop and run - mixed with grass clippings, dried leaves, occasional dried pine needles, garden waste and weeds. I primarily feed fermented feed (just noting because it seems to effect the odor and texture of their stool to a less liquid compound).
Moral of the story is that I rarely notice flies - maybe 1 or 2 every now and then, but never often enough to really take notice of them. I sometimes take my 'Bug-A-Salt' rifle out to wage war on stink bugs and will kill the random fly that I see, which isn't often enough to make a sport of it via the bug-a-salt. Fly 'terminator' style traps go a long way in substantially reducing fly populations if you're diligent in changing them out when they reach capacity.
