How long? How long does it normally take ?
It depends, for my duckling, she got to zip, and then stopped, didn't complete the zip. This was 48 hours since the external pip, I made sure she was not chewing or yawning (to ensure the yolk was absorbed- they yawn and chew with the beak while absorbing)
once she had stopped, I finished the zip and let her pop out on her own.
If they internally pip, and don't progress for 18-24 hours, they need an air hole- otherwise they could run out of air- but you don't want to help unzip until that 48 hour mark, really.
If ever you assist, you must ensure the yolk and blood has absorbed.
This baby was not quite ready. The best thing to do, at that point, would be to put them on something dry and soft, like paper towels, in a bowl or cup in the incubator.