It depends how long all your eggs take to hatch out. A newly hatched chick can go up to 72 hours before it needs food and water, so if your hatch is taking a while to finish, you don't need to worry about leaving chicks in your bator for a couple of days. But if they all hatch at the same time, like Quintip says, you only really need to leave them in there for as long as it takes everybody to dry off and fluff up.
If you have a hatch that's really taking ages, you can sneak in quickly and grab fluffed-up chicks and transfer them to the brooder. You just need to realise that by doing that, you'll be lowering the humidity in the bator, which can take a few hours to build back up again, which can have a bad effect on chicks that are still to hatch out. Sometimes you don't have any choice, but just remember if you CAN leave the bator shut, it's best that you DO leave it shut.