I have an 11 yo almost 12, and within the last 6 months or so, we've been progressively allowing him more time at home alone (he wasn't at all before 11) as well as some time where he is responsible for his just turned 7 yo brother. So, during that time he's actually his brother's babysitter and is to handle it as a job.
Now, we have gone the majority of the day recently. They were home from school due to extreme temperatures., So, I stayed home a little late, left them about 9:00, went to work, then came back and had lunch with them at noon and left them again for the afternoon. But some of the considerations are:
-the 11 yo is very responsible
-They both know the rules for when they are home alone (re answering the phone/ door etc.
-They don't play outside when they are home alone (yet) (that -obviously wasn't a problem at -15F the other day)
-I work about 5 minutes from home.
-Grandma (the good one, not the evil one) lives 1/4 mi away and can essentially see most of our house most of the time
- They have an aunt who could be there in 10 minutes if needed.
So, they have a plan, know their responsibilities, and have options if things don't go as planned. It's been a process to get to the point where we can trust a 3-4 hour stretch.
I don't think I'd let them do all day during the summer, as that's a lot of responsibility for the 11/12 yo and they both have lots of activities to get to anyway, so they need an adult.