Of course, if you're not a very good pet owner, you could leave them alone for days on end. However, hopefully, this isn't the case. Leaving a dog alone every day for over 8 hours can cause separation anxiety, so be careful. Adult dogs will also need to eat twice a day, so you have to feed them. In addition, they need to go to the bathroom, so leaving them outside may be better (if they're okay with the outdoors alone) in the event that you need to leave for a couple of hours. Still, if you're going to be leaving them alone for an extended period of time (over 8 hours), then try to at least have a friend or neighbor check up on the dog and feed it. You also wouldn't want your dog to ruin your house or yard.
A dog can be left home alone for eight to ten hours, but no longer than that. In general, you can leave your dog at home while you are at work. If you can come home at lunch to walk your dog this is better for your dog. Dogs will need to go to the bathroom at least every eight or so hours, and they also need to be fed twice a day.