I was yelling pretty hard!
And, lucky for you I don't have a crossbow, so you can say bolt-bolt-bolt na-na-na-na as many times as you like! hehehe 
So, ok, shooter shot victim on horseback. Victim takes a hard fall, probably breaks a leg or something, but first shot didn't penetrate or wound due to "jack" armour he's wearing. Shooter stands on victim's chest, victim doesn't turn his head to the side, only close his eyes and wait for the shot. It goes through the windpipe. Doesn't have to stick out, but I'd guess victim won't move his head either way with most of the arrow still stuck in his neck. So since a major artery hasn't been nicked, there won't be any blood frothing from his mouth and he won't die quickly. If need be, shooter can take another shot at close range, but that would depend on the impact of the neck shot. So consider only a single shot through his neck.
If he's young and healthy, and found 3.5hrs later. Still alive? Any movement from him would cause some bleeding? He'd be weak but still breathing? Able to speak? I doubt, but you'd probably know better. I also doubt there's much they would be able to do to help him survive something like this. So once they've pulled the arrow/bolt out, his time is pretty much up? How quick would depend on the blood loss?
If all else fail, could always have him step on the victim's head like you've suggested!
Although I think we're going to test K's sensibilities with this 
Thanks a mill!