When can a dog be left for longer than a few hours?

We have a 10 week old lab cross. When she is 12 weeks old, all the kids are going back to school so I am going to hire someone to come at lunchtime and let her out and play a bit. Obviously, she is too young to stay all day without going out but at what age is a dog ok to be left for the day and not have to go out to pee etc?