Never, as long as the CIA still exists. The institutional reputation of the agency is very low right now among large swaths of the population. Imagine what would happen if it was revealed that they had a hand in killing a sitting US president? No matter how long ago it was, their reputation and credibility (and the reputation and credibility of all the institutions and people who continue to uphold them as patriotic defenders of democracy) would be forever tainted (even beyond what it is today).