When my son was almost 12 and daughter was almost 9. Both of them together, not just one.
Usually it was for 30 minutes when I needed to go to the store and my husband was on his way home but hadn't made it yet (he has almost an hour commute). Or if I needed to go into town and my husband was going for a run (a 35 minute or so thing usually).
After son turned 12 or so, he started getting left more frequently and by himself--when I take daughter to piano and hubby is still at work, for instance. That takes about 90 minutes, b/c the lesson is a 20 min drive away. With daughter, we don't leave her alone.
Now they are 13 and 10.
We have left the two of them together in more recent months while we went out for dinner on a Saturday night. Local, but 20 minutes away. We have our cell phones. I actually got a text from my daughter letting me know Dad had left his phone at home and btw, could they make popcorn? LOL
We actually left them for several hours a few Sunday afternoons ago, to go to an event in the next town. A 50th wedding anniversary luncheon. We got them lunch, then gave them rules and instructions and left them to their own devices. Son was wiped out, as he had been at a weekend leadership camp/seminar and had just been picked up that morning before church. So they pretty much just watched Netflix or movies and let the dog out as needed.
It worked out fine for us. The kids have been pretty responsible. No guests are allowed. They stay inside (not playing outside). They usually just watch TV or a movie or play their video games or whatever. Sometimes they are given a chore to do first, like clean their rooms or get their shower before they can start the movie. :)
It really depends on the kid(s) and their levels of maturity and responsibility and capability. And where you live, too, such as if you have neighbors that you know and they can contact if needed in an emergency.