3-4 days a week, they might be home alone for a few hours when I'm working, depending on if anyone else is working/at school. I arrange it so that they are not alone for more than 4 hours at a time... I really don't like leaving them alone that long without a walk, and I worry about Fozzie's SA flaring up. Luckily, I have friends that live 30 feet away who also visit them. I come home on my lunch to walk/feed/play with them, too. Hopefully I'm going to start bringing Fozzie to work so I won't have to worry as much.
I think that a lot of it depends on the dog. I adopted Fozzie with the requirement that some one was home during the day, and that makes sense to me, for a puppy. That was when I lived at home and some one was ALWAYS there. I don't feel it's fair to leave a puppy alone for 8 hours with no potty breaks or interaction whatsoever. But, there are lots of dog walkers, daycares and other options out there!