I think this is very much a matter for judgement taking into account the circumstances and the child. I've left DS7 alone for as long as it takes me to go to the corner shop (10 mins or so) a couple of times and actually wouldn't mind leaving him for longer if he was happy with it - he's very cautious and not likely to get into trouble - but he's not completely happy about it - for the same reason! - so I don't.
In the UK there is no legal limit, but parents are responsible and it can be child cruelty, IIRR, if the courts reckon a parent made a wrong judgement. There was a story in the UK press a few days back about a woman being officially cautioned for leaving her 14yo in charge of her 3yo for 30 mins. Unless there's a lot more to the story than meets the eye that's crazy, and the moral seems to be "don't accept a caution without legal advice" - had she refused and insisted that the police prosecute her if they thought she'd committed an offence, surely she'd have been found not guilty. At any rate if I'd been on the jury! But some children's charity apparently "recommends" that children aged 13 and under never be left in the house alone no matter for how short a time. Madness. One of our regular babysitters was 12 the first time she babysat, and DS was 4. I think it's unusual for that to be sensible - she was unusually responsible for her age, DS was unusually cooperative for his, they knew one another, we weren't far away and we were in mobile phone contact - but I have no doubt that it was sensible.
