If you read incident reports, which are published by the FAA, there are cracks all over the place in aircraft. Most of these are found at relevent C or D checks when the aircraft is given a really thorough inspection. It really depends where they are as to how significant the consequences can be.
The DC 10 crash in Chicago in the 80's was caused by a (from memory) 7" long crack in a 14" long fitting in the rear engine pylon attachment.
It was determined on subsequent investigation that aicraft flying around with 3" long cracks were still doing OK, it didn't cause a problem untill the crack propagated to about half the length of the bulkhead.
After scanning the above linked report, I would say that you are never going to find a definitve answer to your question.
Just be thankful that the airline industry in all it's facets is a lot better now than it was back then.
I remember seeing another report on aircraft losses and the rate of loss today has gone way down from the earlier years when things were just being developed. The 50's data was unbelievably high.
Good Luck anyhow.
JohnB 35 years in the Aicraft Manufacturing business.