Hatcheries put chicks in the mail the day they are hatched and they arrive alive and well 2 and often 3 days later. Mama hens will keep the first chick that hatches in the nest until the last one to hatch dries off in a normal hatch. You have at least 48 hours and probably much more from the time the first one hatches until you need to open the incubator. I'd wait 60 hours without a worry, probably longer if I saw activity still going on.
When a chick gets hungry or more likely thirsty, its cheeping will change from a the normal here I am cheep to a loud, shrill, aggravating, attention-getting, constant, distressed cheep. I don't like to wait that long. If that happens, it is time to take the chicks out.