I don't know about unconscious-in-a-horse, but I do know that unconscious does not equal limp. If they're like smaller livestock, the animal will convulse even after it's dead--- it just doesn't know that it's dead yet.
So that's one of the reasons why you try to restrain an animal. Cutting its throat is good, because it desanguinates the carcass. But living things prefer to stay alive, and it will put up a fight, and its body will continue to fight even after its consciousness isn't there--- or even after its head isn't there, for that matter. A lot of it is probably reflexive.
I've never timed it, but for a rabbit, if I had to guess a random number, it might be 45-60 seconds before I'm confident enough to release my grip on it. For a larger animal, it might take longer-- because it has more mass to it-- or it might take the same amount of time-- because the wounds are proportional. So I couldn't make a guess there. I don't do the cutting, but I do find that even though rabbit skin is thin compared to horsehide, it's not easy to cut through, even with a sharp knife, because of the fur. So if your bandits have good weapons that are well-cared for with a good edge, they might be able to make their cut with one good stroke. But if your bandits have dull, nicked blades--- it's going to be a process.