I have a dog and have, during his lifetime worked full time. He was trained for about two weeks before I returned to work having acquired him from the RSPCA. His yard had access to a deck which permitted him to look over the fence and watch the world go by. It had a wading pool that was left filled with balls of various sizes for him to play with. The cat run extended into his part of the yard so he could interact with the cats. He had a wow of a time all day while I was at work, and when I got home we would pile into the car and go to the dog park where he got to play with his buddies (regulars at the dog park). After, we would go to obedience training if it was that particular day of the week. Then I moved to NZ where he would spend his day in the yard (with various enrichment toys), being visited by the cats, and chatted to by the neighbour through the fence, and in the evenings we would go up to a huge dog park where he could run and chase smells through all manner of long grass, or we could go for long walks along various tracks.
Then I moved to the UK and was away for 12 hours a day what with the commuting. On my return we would go for a long walk again. And if I knew i would be delayed even further, getting home, I would arrange a dog-walker to come take him out for a round or 200 of ball-chasing.
He is nine now. And he is a well-adjusted, happy dog who, now I am home all day gets rather less exercise than he did in all those previous years. I think, rather than have hard and fast rules, it is probably better to consider the nature of the dog. Mine is, and always has been, perfectly happy with the arrangement. If that were not the case, he would have howled the neighbourhood down. And I am assured by various past neighbours that he does not do this. And his lack of destructive behaviour shows that separation is not a problem for him.