How long to break on through?

[doodle chicks]

Hi can anyone help me as my wife is starting to panic.
We have 8 eggs (6 French Marans and 2 Manxies) in our home made forced air incubator, 2 of which have a small crack which appeared on day 21 but nothing else has happened for 12 hours. The crack is the same.
My wife is tempted to help things along which I have manged to prevent so far.
How long does it take for the chicks to break on through to the other side?
All the best S

 

[incubatingisfun]

a chick should hatch out within 24 hours of the first pip

 

[doodle chicks]

Thanks Matie, have a good day. S

 

[incubatingisfun]

you too