It's rather immaterial in a certain sense, since I have no idea, and can't give you Book, Chapter, Verse.
But, generally speaking, I would offer the following supposition:
As long as it takes for their punishment to be meted out. Some receive many stripes, as it were, some receive few. I submit this suggests a degree of severity based on how much or little a person knew of the Lord Jesus and the Gospel.
For example: regarding Lazarus and the Rich Man. One argues it's just a parable. Another says it was a real story. In truth, it doesn't matter either way.
While the rich man was in torments, his punishment was still being exacted. When it was filled up (See
Matthew 23:32), he would die the second death and be no more. Prior to that, he was experiencing conscious torment. He was, as it were, receiving his "stripes" until he died eternally.