If you have actual pips and not just peeping from inside the shell, it can take up to 24 hours before a chick will transition between pip to zip in which the chick actually hatches. After the pip is made a chick rests, and strengthens it's lungs breathing the outside air and most importantly the vascular system between the egg and chick has to shut down. This can take as little as a few hours or as much as a whole day. Also, if the yolk has not fully absorbed before the pip, that has to happen too for the chick to be fully ready to hatch out. The worst part of hatching is the time between lockdown and that first pip and the time between pip and zip.
If you are just hearing peeping from inside the shell with no external pips, it could take 24 hours before the external pip and then another 24 to zip.
My eggs average 12-18 hours between pip and zip. Sometimes I get chicks that hatch quicker, sometimes later.
