I would be about 20-30 minutes.
I've just run out of washing powder and I know my 8 year old will whine about going and he constantly asks if he can just stay home with his brother (12, nearly 13).
My parents used to leave us kids. In fact we used to baby sit our baby brother while they went to the pub. But then, a lot of the stuff they did back then was questionable by today's standards. 
AIBU to leave them while I popped out?
(Wtf it's not letting me do the bloody poll?!)