I wouldn't, I left mine for just over 3 hours and they dug their way out! It took me a good couple of hours to find them luckily they were still in the garden but if the neighbours dogs had got into our garden I think things could have been very different
It is safe so long as they have shelter, food and water. You also need chicken wire to go around the perimeter to stop digging out. My sister left our rabbits out in a run with no shelter and no water on an incredibly hot day when I was on holiday. The black one died of heat exhaustion, the white one survived to be looked after the following year by her... She decided to create a run from old wood pallets etc instead of using his normal run. He escaped never to be seen again!
They will dig and they can dig quickly. Mine are out in a run attached to the hutch but I've dug down under the turf and layed wire. However I didn't think to bring the wire up verticaly at the sides and of course they escaped. One day there was no sign of digging, next thing I know there's a full on tunnel and they're off down the street.
You need to bunny proof it properly first, but then yes I'd let them out in it while at work. Mine are.
I have two, think it must have been a joint effort.
After I'd done the wire my (too smart) DD said to me "but mummy you didn't make the wire go up so they will just dig between the run and the wire". She was 7 at the time. I assured her it would be fine and they were for 6 months. Then they were out twice in a week.
Ours stay outside all the time. Their hutch is attached to their run. We used to have netting ont he bottom of their run to stop them digging out, but don't anymore, as they are obviously not into digging.