If we are only talking about analysis of his foreign policy, especially regarding the fall of the Soviet Union, then we see disparate analysis of his administration because of the political debate that we continue today. Some conservative writers (I wouldn't call them "historians") argue, for example, that he single-handedly brought down the Soviet Union with presidential policy. Other analysts of the fall of the Soviet Union (who I would call professional historians) will ignore Reagan altogether.

I think the clearest example of modern political debate influencing analysis of the past is in the example of FDR's New Deal because, today, people believe different things about how government intervention in the economy works. On top of it, it's very hard to discern definitively whether Roosevelt hurt or helped the United States leave the Great Depression, or if his New Deal failed the play the decisive role either way. If you believe that the government should expand social programs today, then it would be very difficult, no matter how hard you strove for objectivity, to not see greater benefit to the New Deal because of it.

This is one of the challenges historians face, and it is one of the reasons that /r/askhistorians implements the 20-year rule: the closer in proximity we are to an event, the more likely we are to allow our personal values and political beliefs influence the interpretation of that event. However, there is no special amount of time that needs to pass before historians can look back on it objectively. Political history sometimes seems especially problematic in this regard, because we continue to have some of the same political debates today that we had in the past.