Question:
My viewpoint character is about to get bludgeoned in the head by a blunt makeshift object such as a branch, baseball bat, golf club, etc.

He's going to blackout but it won't be a life-threatening injury, just a concussion and a splitting headache once he comes around. Unfortunately, I need him blacked out in the 10-12 hour range. Being out cold for that length of time seems (to my layman brain) a much more serious injury than a simple 'bonk' on the head.

Could a simple, non-threatening head injury lead to unconsciousness for up to half the day?

Answer: It is possible for a head injury to leave one unconscious for quite a while (a few hours at least) without any life-threatening complications. I've found a technical introduction to comas at According to it, "In patients with traumatic coma, the length of coma does not necessarily imply a poor prognosis. The outcome from head injury depends primarily on the level of consciousness at the outset of injury and on the age of the patient."

It is also possible to be killed by complications from what seemed like a very minor head injury. Symptoms could take hours, days, or longer, to appear. It can also be very dangerous to get a second brain injury while recovering from the first; this is called second impact syndrome.

What is the purpose of having the character unconscious? What does he do when he comes around? Will he receive medical treatment, and at what tech level?

If he's going to be treated by a doctor, he could be kept unconscious by a depressed fracture, and recover quickly once it has been elevated and the pressure on the brain relieved. A character in Dracula (1897) got a depressed fracture, which (if I recall correctly) was then elevated by a doctor who happened to be in the house, while Galen (at a much lower tech level) called for removal of the bone fragments using fingers or forceps. This would be a handy way to have him make a quick recovery after being out for hours, though I suppose it will leave him with a weak area on his skull that would be easily broken if he was hit again in the near future. He also ought to be observed for a while in case complications develop.

If he's comatose due to brain injury, rather than pressure on the brain, then he will also be semicomatose for quite a while. He might go through a stage of having incomplete use of his muscles, and he will be tired, disoriented, moody, nauseous, etc. If the purpose is to keep him out of the action then he could be comatose for minutes, semicomatose for an hour, and conscious but nearly helpless for hours afterwards.