Int to ext pip 24-48hrs is normal. For me I don't do anything until external pipping. Then each phase gets 24 hrs. If ext pipped and no progress for 24 hrs, I carefully enlarge the pip hole only along the air cell where there are no blood vessels. Assess for energy and effort. Keep the membranes moist. It can take a long time to absorb the yolk so I don't rush things. If the chick is moving and peeping, I let him work. But if he seems stuck at a phase for more than 24 hrs, I help a little. A little chipping here, enlarge the pip a little there...slow and gradual.