I think St. Dysmas is a good example of someone who received a plenary indulgence. He was being crucified and declared that he deserved it. He must have given up all attachment to sin, and his only concern was that Jesus was innocent, and he had no concern about his own fate.
So in St. Dysmas, we have an example of a saint who lived a sinful life up until his last moments, where he was redeemed and went straight to heaven.
When I'm attempting to gain plenary indulgences, I always offer them to my deceased loved ones. Otherwise, I would feel they were worthless, as I know I'd ruin it within a matter of hours.