Since 1989, DNA evidence has exonerated 255 people in the United States. The average prison-term length among them? 13 years.Â In some cases, even with exonerating DNA evidence, the wrongfully imprisoned donât just walk out of prison. Former Ohio attorney general Jim Petro learned that sometimes, even when the evidence points toward not guilty, the innocent are prosecuted and serve time. He learned this the hard way, as the first sitting state attorney general to intervene on behalf of the accused in a case to exonerate them.