My dd, now almost 13, wanted to be home alone when she was about 10. So, I would let her stay by herself if I was going to be in town, and home within an hour or so. Then, it was for a longer time after about 6 months. She did fine. At almost 12, we let her stay by herself for indefinate periods. Now, she is staying alone a bit at night. We always bring a cell phone with us at night though. She has never called us, but likes knowing that she can. I think the longest she has been by herself at night is about 3 hrs. so far.
Rules....since we have caller ID, she can see who is calling and make a judgement call as to whether or not to answer it. Doors stay locked and unopened. No using the stove. No leaving the house to play in the yard.