My kids are 4 and 6 now, and are pretty independent. I've left them alone before with their iPads on, and they've happily played MineCraft without complaints. But I'm never too far away (I work near my house), and check on them regularly.
I'm not sure how old they'd have to be before I left them alone for a few hours, and wasn't nearby. Maybe when the kids are 8 and 6 years old, I'd start considering it? That's way younger than what I would've considered in the past, but the area we live in now is pretty safe, so that's a factor as well.