"Should" does not always match reality.
I've had hatches in an incubator and under a broody where all the chicks hatched within 16 hours of the first one hatching. I've had hatches in an incubator and under a broody where it took a lot longer. One hen hatched a chick late Monday. She brought her chicks off of the nest early Friday, about 80 hours later. I don't know when the last one hatched but it was well after 24 hours.
One incubator hatch, a chick hatched fairly late in the day. I saw no activity in the incubator until about 24 hours later, I saw a couple of pips as I was going to bed. The nest morning when I got up the remaining 16 chicks had hatched.
This stuff is too random for hard fast deadlines and limits. With living animals a lot of different things "could" possibly happen, regardless of what somebody thinks should.
Before a chick hatches it needs to do a lot of work. It needs to dry up the blood vessels in the membrane surrounding it. It needs to absorb the yolk to live on before it eats and drinks. It does something to the gunk that is in its down so when it dries it is all fluffy instead with that down matted. I don't know what else they do. Some chicks do a lot of that between internal pip and external pip so they can zip and hatch pretty quickly after external pip. Some do a lot of that after external pip and before zip. These can take an excruciatingly long time to finally zip. When they finally zip they peck a line around the egg so they can push it apart and finally come out. This "zip" usually happens fairly fast though one might get tired and rest a bit partway through.
If you try to help one before the blood vessels have dried up it can bleed to death when you break those unwrapping it. If it has not absorbed the yolk when you help it, it might still absorb it or the yolk may break or get messed up before it can be absorbed. Sometimes you save them when you help, sometimes you kill them.
Don't give up on them until you know it is dead. Some of them can take a very long time.