Left for 8 hrs means the dogs are left for 8 hrs without seeing anyone in a run that's at least 10 foot by 20 foot.

Everything else being perfect means just that. House almost in the middle of nowhere (the road/track stops there), surrounded by open fields, forest and moorland - morning/evening walks on the doorstep. A lawn the size of half a football field with the facility to light it up. Nearest neighbour is approx 2 miles away. Dogs are an inclusive part of the family after walks/dinner etc (not isolated in the "dog room").

When at home, it's a country, outdoor lifestyle with the dogs and/or horse (kept in one of the neighbouring fields).

It depends on the dog. My parents dog is left 9 hours alone, 4-5 days a week and she's as happy as can be.

IMO there is no way I would leave my dogs for 8 hours even if they have an idyllic lifestyle the rest of the time.

Would it not be possible to have someone come in and check on the dogs?

If someone was going to be checking on the dogs half way through the day (or taking them out to stretch their legs/toilet breaks) then I cant see a problem. As this is what I will be doing when I return to work in July.

But if the dogs day is not going to be broken up, then Im sorry to say that I think its far too long. 6 hours is the maximum id leave without a break for them.

How would you or your parents know if the dog was happy if nobody is there for the nine hours per day? What can be seen is how the dog behaves when somebody is there.

I think it would be okay if someone could pop in to check on the dogs during the day, let them out for a quick walk or leg stretch and wee break at least.

I also think it would very much depend on the individual dog (and breed). I wouldn't advise putting a pup into that situation, but a dog that was perhaps already used to being left and was happy with the other dog company, then it could work.

Could you tell us a bit more about the dog in question, and the dogs that already live there? What breeds/ages are involved?

No, sorry, I don't think that is acceptable for any dog.

Eight hours is far too long without being checked on, given some food and fresh water, etc.

And again, one would have to be extremely cautious about leaving three dogs together for that length of time. It is not unheard of for dogs to turn on and even kill one another. There was a very long thread about it not so long ago on this site, very upsetting for the dog owner.

You say that everything else is 'perfect' but IMHO that is just not the case. For the bulk of the day, nobody is supervising the dogs and the kennel run is not that big. Hardly a 'perfect' lifestyle for any dog, again that's just my opinion, as you've asked.

One time we had to leave Dahlia for 9 hours. We asked someone to come check on her to make sure she was ok and see if she wanted to go out. They stopped in, she was on the couch asleep. She didn't get up. They got out her leash and asked if she wanted to go for a walk; she stayed curled into a ball asleep. Ultimately they left her there as she seemed rather content to lounge on the couch.

My parents once set up a video camera and taped about 4 hours of their dog's time alone to see if she was bored, see what she did. She curled into her bed and slept most of the time. Occasionally she got up to bark at someone walking by the house, but most of it was spent asleep.

Unless a dog has separation anxiety or some other issues, most of them will just sleep the day away.

Alternately, dogs can be given some sort of puzzle toy or a kong filled with frozen stuff which keeps their mind active for a time and will help make them even more tired.

They video taped her once to see what she did. She looked rather content, curled into a ball most of the time. Now because it's her routine, she often curls up and sleeps during the day even when someone IS there.

Besides that, she is a happy little dog in the evening and morning, is not anxious, is not worried about being separated. I think if she were anxious about being left alone, there would be some sign in the evenings.

Curling up and sleeping is not a sign that the dog is content...it may well just be that it is resigned to it, that is VERY different.
My rescue girl was always kennelled and would happily sleep all day and doesn't ever,ever demand exercise. She would sleep and not go out without a murmer because that is what she is used to, I actually find that really sad.