The answer really depends on who is answering the question. My personal rule is, I do not assist unless the chick is strong enough to pip externally. I've never had any good outcomes from assisting a chick that didn't externally pip. Most died right away, others died after a week or so.
For an external pip, where they pip helps determine how long I wait. If they pip on the correct end of the egg (the fat, air cell end) I help after 12 hours. If they pip on the wrong end (narrow end without the air cell) I wait 36 hours. The reason is chicks that pip on the right end often pipped internally a day or so before and are closer to hatching. Wrong end pips are usually earlier in development and need more time in the egg. One thing you can do while you wait is make sure you can make sure the chick can breathe through the hole. I peel back the tiny shell pieces around the pip and make sure I can see the beak and nose holes. if you have humidity problems, sometimes egg fluids can cake up around the chick's beak and nose and suffocate it. If there is gunk, get a moist q-tip and clean around their nares (nose holes). If I do start helping I peel back the shell in small sections looking for veins in the membrane. If there are veins, the chick isn't ready.
If the chick has started zipping and stopped for more than an hour or so, help right away. This usually means that the chick is ready to hatch but the egg fluids have dried and it has gotten stuck to the membrane on the inside of the egg and can't move to finish hatching. This can happen often with incubator hatches if humidity isn't right during the whole of incubation or if your incubator dries out during hatch.
As for ones that don't pip externally or internally. I candle a few times to see if there is still movement in the egg. If there isn't movement, I wait another day and then give up.
