Depends totally on the child, I think. If you're really confident that he is sensible, wouldn't let anyone in or do anything you've told him not to, then I think it's fine. What about eating? Won't he be hungry when he comes in?
I'm happy to leave my 9 year old at home alone for that length of time, but she has to stay downstairs (so she's not trapped in remote likelihood of fire) and she's not to eat if I'm not here (my paranoia about her choking, although she could choke and die when I'm upstairs, frankly). DD knows all the drills of what to do in case of emergency. She is a child who will quite happily sit on the computer or with a book, not moving or her attention wavering, for hours. I'm soon going to be doing something that will involve me leaving the house suddenly during the day (DD is home educated) and being gone for between ten mins and an hour and a half (max) and I'm happy to leave her for those amounts of time as long as our contingency plans are in place.
I'm sure most responses to this thread with tell you not to do this, however - MN is a very paranoid risk averse place. 
Ultimately, you have to make this decision based on the child that you have. Also the house you have, the neighbours you have, the neighbourhood you live in, your perception of risk etc. Mainly though, it's about what you know of your own child - noone here can make that call for you.