We have been in the countryside two and a bit years. One year with horses in front and behind us, now on a dairy farm.
They never really go away. There hasn't been a month in our 14 months on the farm fly free, although they do calm down substantially from Dec to March.
What tends to happen is the type of fly changes. One month big fat ones that want to land on you, the next one teeny ones that just want to fly around lampshades.
Thankfully, I had a word with the farmer and she kept the cows out the field next to us this summer, but that doesn't stop the normal fly level, which is to say they drive me batshit. Last summer 50 in a room within an hour or two, this summer ten. Given that we have had no summer, well, I have been quite thankful for that actually.
Horses or cows, I just think it's a countryside thing.
We have changed our behaviour. Keeping windows closed (which they still crawl through because of sash mechanisms), never ever leaving out unrinsed plates, not leaving the dishwasher door open with dirty stuff in ever, having a dozen fly nets so even if I am just making toast and go to another room between doing two lots, not even a buttery knife is left uncovered, and any drink at any point in time has a coaster on top of it.
Do you know where they are getting in? They're not horse flies, are they? Those nasty buggers can have you in A&E if you are bitten by them - we always stayed well away from the horses when they were hanging around them (and it was horrible to watch the horses suffering).
I have rigged up my own DIY fly traps on light fittings because last summer in the old house the OH kept walking into those brown strips (which melt and get ever longer in the heat) and he was losing it. Transparent fly strip, hole punch, nappy pins, string. They are quite gross, effective in some rooms, not in others. They can also just be stuck on windows.