Generally speaking you will NOT want to intervene in the hatching process when incubating fertile eggs. If conditions in the incubator are right, it can take 24 hours for a chick to escape the egg after it has pipped, and that's perfectly natural and not a cause for concern. Often it takes much less time, but 24 hours or so is fairly common, too.
If it's been more than 24 hours since the chick first pipped and you have had problems maintaining a high enough humidity during incubation, the problem may be simply that the chick is stuck to the dried out membrane inside the shell. In that case, you may decide to help, but be forewarned! It is VERY easy to cause the chick to bleed to death when you try helping him out of the shell. You don't want to watch that happen, and feel that responsibility. Further, opening the incubator to assist this one chick lets even more humidity out of your incubator, and makes it more likely that OTHER chicks will have problems escaping the shell. This just isn't a decision to make lightly. It's easy to cause more problems than you had to begin with.
Some people choose to help whether they think the problem is humidity, or whether they have no idea what the problem might be. We understand! If you're soft-hearted, it's natural to want to help. You've been waiting all this time--21 days!--for those babies to hatch, and you want a happy ending. You don't want to just sit on your hands while someone struggles if it's possible for you to help.But what you want to do isn't always what you should do, even though we do understand that helping is a difficult temptation for a compassionate person. Before making any decision, though, be sure to consider the risks carefully.
If a hatch is taking excessive time, it's possible that there's something wrong with the chick. Before you do anything, take a little timeand determine if you are prepared to provide extra care in a case where the chick you helped to hatch is ill, disabled, or otherwise has special needs. Special needs chickens can often become the family favorites, that's true. But if you are not successful helping, there's a load of guilt you'll carry with you. And if you ARE successful in helping an ailing chick out of its shell, it can be a long term commitment, and may require veterinary visits and so on to maintain a reasonable level of health. The chicken may also require special accommodations.Just be sure you're fully prepared for the consequences of taking action.