Have you read through the article about assisted hatching?
https://www.backyardchickens.com/articles/guide-to-assisted-hatching-for-all-poultry.72886/
My experience is much less than the author, but I would say: later than you feel like you should. Give the wrong end pip 24hr. A wrong end pip is the internal and external pip all at once. Normally a chick would do one, rest 12hr or more, do the other and rest again, and then hatch.
Every time I think I might need to assist, it's usually too early.