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As with human pregnancies it seems on rare occasions they come early, but more often they come a little late. Most of mine that have pipped go pretty quick <12 hours, but more than a few have gone nearly 48 hours and made it out fine (a couple a little longer). I've lost more than saved by 'helping', so it's still very hard for me to decide what to do. But one helpful thing to remember is that they've got a nice sized yolk sack attached which gives them energy for at least 48 hours once they are ready to find their way into the world. If you see that they can't get through the white membrane, chances are they are going to have a rough time and I would read carefully and thoroughly the 'sticky' posts under incubating and hatching in the chicken section before trying to help.
I know we all have slightly different advice - and it can be a hard road. Nature IS amazing, but for some it's a pretty daunting task finding their way out of that shell.