They usually zip around the edge of the air cell, but from pip to zip can be a while because they rest for quite some time after pipping usually. If there are still veins in the membrane, then you probably will have to wait and watch... if the membrane you pipped for it was a pale brown or cream color with no veins (wet the membrane with a q-tip and see if the veins light up red), then it should be able to zip at any time. If it were my egg, I would be watching it like a hawk in case it got stuck somewhere it couldn't breathe and I would probably help create a zip line for it (assuming it was safe to work, meaning a 'dead' membrane). If there ARE veins, just leave it be as long as you can see it breathing. Veins indicate that it has not finished absorbing the yolk and is not ready to exit the shell.