Can you help with info..Baby rabbits?

I have four baby bunnies and i have started to allow them out of their hutch on an eveningtime.They have a 6" hutch which is raised up ontop of another hutch.Thay cannot climb into it and are reliant on me.The question i nee the answer to is How long can they stay out and play without food and water...At present i allow them half hour , can i extend this play time.....Iknow that they have a hutch that is big enough for exercising but i like them to be with us on a night time.........