As long as you leave them there! If all your chicks hatch at once, you can take them out whenever you like. Most folk like to wait until they're all fluffed up and dry though. If you have some hatched and some still to come out, you should try to wait until the hatch is finished and everybody's out of their shells. If you open the incubator while some are still hatching, you risk drying out the membranes and getting the chicks stuck in the shells. Chicks can go up to 72 hours after hatching before they need food and water, so if you're having a long drawn out hatch, the early ones will be fine in there for a day or so...