I have left my dogs outside for a couple of hours at a time and as adult dogs, they are just fine. My sister once left them out all day and they were fine except she didn't leave any water for them. (I could have killed her.)
My pup, Rider, is a different story. Even with me out there with him, he can get into all kinds of mischief. It takes training, a watchful eye and lots of supervision to show the dogs what is off limits and what they can do. But it is possible.
As boxers are not really outdoor dogs, I tend not to leave them out more than about 30 minutes at a time in hot or cold weather. If it is pleasant enough weather, I leave them out until they want to come in (usually about 30 minutes of play, then they want to come inside).
My adult dogs never seem to want to stay outside for long periods of time, so I see no reason for them to. I enjoy having them inside with me.
Dan