At what age would you leave your DC alone for 75 mins?

[LittleDoritt]

I think my DD is too young still but I'm curious as to what others do.
The children's activities have swapped around a bit for this academic year - for the worse. Although they don't have scheduled activities every day, their sibling has something on the day they are free which means being out of the house for a period of time every single day after school. No day to just come home and hit the sofa.
It got me wondering at what age I can let the elder DD chill at home in front of a film while I ferry the younger to ballet, music etc.
DD is (just) 9, and we would be away from home for a maximum of 75 minutes.
What would be your minimum age?

My child is the same age and I'm thinking I could start to do this now. She is very independent and happy with the idea.

does she have access to a computer tablet etc so she could email you and you can set your phone so you set audible prompt if email arrives
unless in a very safe community where she knows neighbours really well I would get her a phone I would leave DD almost 10 for about an hour or so but she does have a phone but she would also know not to tell anyone she was home alone we live in a very safe place

We did it from age 10. At that stage DS got the bus home by himself and one of us would generally be in the house an hour and a half later. He was less keen when it got to winter though so the dark makes a big difference.