It could take them 24 hours or more to hatch after pipping, but thankfully most of them don't take that long! What I do if my chicks don't make progress and I'm sure they are not going to be able to hatch unassisted (for example if they are malpositioned and cannot turn) is carefully open the pip hole just enough so I can see the membrane. I then dampen that membrane with a wet Q-tip to see if there are any active blood vessels in it still. If there are still vessels I out the egg back in the incubator and wait 2-3 hours. Then again I dampen the membrane and check, until I don't see any more vessels, after which I slowly open the shell more, dampening the membrane as I go, as sometimes you may not see vessels near the pip hole, but there will be more "deeper" in the membrane, especially near the chicks belly. Be very careful when you get to this area. If you are 100% sure the blood and yolk sac has been absorbed you can remove enough of the shell for the chick to get it's head and feet out and remove the inner membrane so it can hatch by itself.
But for now make sure the chick can breathe and is o.k. in there and leave it. Just check on it now and then. The longer the chick stays in the shell the longer it has to finish absorbing the blood and yolk and the better the odds of it being ready to hatch when you start assisting. It's just much easier this way! If it hasn't made progress after 18-24 hours it will probably need some help and this is where you start assisting.