No hard fast rule - depends on the dog. I was very concerned getting Opie that he would be unhappy being an only dog, since he lived with others in his foster home. But he is a pushy big boy, demanding of attention, and he has settled in beautifully being an only dog. He does not have SA, of course a bit at first, like all new dogs. But as soon as we did away with his crate (like the first week!) he was fine. My concesssion to him was to let him sleep on the couch. I originally didn't want animals on the couch (silly me, didn't know the joys of cuddling with a greyhound!), but since he is such a good boy when he is alone I just cover the couch with a flannel sheet.
Opie gets lots of stimulation - we play with him, he goes on long walks each day, and to the dog park at least once a week. We take him everywhere, even just to go to the store for a ride. And yes, we do work, but usually are not gone for longer than 8 hours, usually less. DH is also in a band, so oftentimes on weekends we will be gone on a weekend night.
So if you are matched with the right dog, should be no problem.