It will take longer during hunting season as there is more viscera aruond to feast on.
Between the buzzards, coyotes, ants, rats, maggots and mice. I would think it will be a month or more before you will be able to drive by those remains wthout a frgrant reminder.
Maybe you should try animal control or the like, if it has a brand it might be able to be traced back to its owner and they can pay to clean up their mess.
If the head is still intact, take some photos of it and circulate them with the story. Someone, perhaps a vet that had worked on the animal may reconize it. Some news networks or papers thrive on picking up on stories like this.
If the smell is not disrupting your normal activities, perhaps it would be best to let it be.
Depending upon the jurisdiction you may not be allowed to bury the carcass.
Once the flys get to the carcass it is just a matter of a couple of weeks before all that's left is hair.
I'm thinking I might scoop some dirt from the bank on the opposite side of the road, which goes uphill, and pile it on the horse. Maybe it will mitigate the smell somewhat. I would say that it is around 700 feet from the house, downhill and likely downwind.
I'll sleep on it tonight.
I hate the way the smell seems to stay with you for some time after you leave the scene. It's like it gets into your mustache or something.
No way to call your local fire department for a quick controlled burn? I'd thing some beers would help the motivation. Water truck wets down area around the horse then burn it up. Fire team on standby.