With older horse, could be a while! Our old mare WOULD NOT get near cows at Clinics, watched them close riding down the road, but “knew the fence would protect her” from the galloping heifers coming to see us. So never pushed HARD to be cow accepting.
Well DD wanted a calf for 4H, and she got it for Christmas. He had a paddock between the two groups of horses, so he couldn’t get injured, was handled daily so Old Mare HAD to have him nearby to eat her hay. She was terrible for the first few WEEKS, running away instead of eating even. Hunger won, she got close to eat out of the farthest feeder from calf. THEN the face making started!! She had the most threatening expressions on her face, teeth showing like an Alligator towards the happy calf. He didn’t care about her faces!
It took a long time for her to just be “accepting” and not worked up over it. And this mare has YEARS of experience facing down all kinds of situations, not spooky or a “normally” silly horse at all. Just hated cows. I did start adding a cow pie in her clean stall daily, where she couldn’t run off from the smell. I do think that helped some. Some cow pies in your horses paddock, stall, might help if she is not in view of cattle at all times. Gets her used to the smell, to associate it with live cattle smell in a herd looking at her.
If you get really desperate, you could put her beside a bovine, in adjoining paddocks for close exposure. Just remember that horse KNOWS each bovine is an individual! So they KNOW that there are “general” cows and then there are Watson and Ruby! Horse CAN tell each bovine apart!! Our Old Mare had to learn each calf as a new experience, but it took a much shorter time than that first calf for her to “settle”. She still doesn’t like them, but not a big enough deal to run from anymore! The geldings LOVED the calves, had morning “run to the field” races every day the calves came out. Horses on one side, calves on the other side of fence, and would go neck-n-neck down the fenceline. Calves usually lost because they would start kicking up their heels instead of running.
Time helps, but how fast you want her over it, may mean some closeness by stalling or paddocks beside each other.