How much time is enough?

We hear on the news all the time, That some person commits murder. killing old people, young people, little kids. Then we hear the criminals who commit these crimes are caught and brought to justice, they are given the death penalty. But then they stay on death Row for many many years before they are executed. Unfortunately, some live the rest of their lives on death row without being executed. My question is how much time is enough time for a person to be on Death row before being executed after being convicted, for the crime they committed?
How much time is enough?
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