It can take as many as 30 hours from pip to hatch. 12-24 is quite normal. They have to completely absorb their yolk sac before they come out lest the others pick at it, or development be damaged.
Maybe you aren't even tempted, but I know what it was like when I started, so I'll warn you anyway.
I don't recommend helping chicks hatch, although many disagree with me. Exiting the shell is the first test of the genetic quality of the bird. Don't get me wrong a lot of other factors play into how easy it is for a chick to hatch, but healthy chicks should be able to get out on their own if you incubation was normal. Assisting a hatch is something that has only a reasonable success rate for an experienced hatcher, so if you do end up doing it, don't be upset if the results aren't great. You must also be prepared to euthanize any chicks that you help hatch as they may have developmental problems that are and always were going to be out of your hands.
Assisted hatching is a technique and a reasonable skill can be attained, but you will always lose some of them, and like I said hatching is Darwin's first test for these guys. There is a lot of good knowledge on assisted hatching in the archives so peruse it first, should you decide to try to help.