I assume it varies with caliber and charge.
Is there a general rule of thumb or do cases need to be inspected for fatigue?
Yeah, it depends.
After the third trimming.
(For .45, until you loose it. For .38, until the case splits.
"As was the man of dust, so also are those who are of the dust, and as is the man of heaven, so also are those who are of heaven. Just as we have borne the image of the man of dust, we shall also bear the image of the man of heaven. "