I don't know. I operate mostly on blind guessing, instinct, and 'feelings'....
It seems to me if there is a pip then it does no harm to make an opening in the outer membrane big enough for air to get in if it has been 'long enough'....
I tend to open it up when I see the beak. and I try to look if they are stuck in the inner membrane. Sometimes I open that up a little. If there is any blood, I stop.
There is so much that can go wrong messing with them that I wait. It is not unheard of to get a pip and see no more action for two days.
Half of the folks say to leave them be, and half say to help and I'm on both sides.
I suspect they are a lot like human babies and that you never know for sure how long it takes for it to be 'done'.