Wait as long as you can before helping. It really is best for them to do it on their own, it gets their legs working properly. A lot of chicks that are helped have leg problems or weakness for a week or two after hatch which need extra help or correction.
A chick can pip the shell & do nothing for up to 48 hours than come out just fine. They pip to get air in so they can breathe though they may not be ready to hatch yet. Some pip then just bust right out.
(Fun fact: So far with all of my hatches (about a dozen or so) the first one to hatch has always been a boy. Strange, huh? And my silkies pip in the morning then refuse to hatch out until after I'm asleep, in the dark. I rarely get to see one come out)
Anyway, I know it's hard not to jump in there and help, but I'd really only start to worry 48 hours after pip if nothing happens. Some pip and just don't make it, it's sad but it happens. You can candle the egg to see if it is still moving in there by sitting in the dark, putting a paper towel tube to your eye and the egg at the other end, and shine a strong flashlight up through the egg pointing the flashlight straight up through the tube vertically. Sometimes you can just point a strong flashlight into the incubator right into the pip and the egg will wiggle. Try not to open the incubator if you can help it, it lowers the humidity and makes it harder for the chicks to hatch. Also, check unpipped eggs to make sure the chick has not pipped on the bottom of the egg, it may drown if it does this. If you find one, just turn it pip side up.
Hope this helps!
e
