At which time point the questions belongs to the History, and not to the Politics stackexchange?
Is there a particular cutting point, for example 1989/91?
I agree with the answer by Joe W that a fixed cutoff date would be counter-productive.
For me, the criteria about whether a question belongs on Politics or on History is whether or not the answer is still relevant for the politics of today.
For example, when most of the people involved in an event are no longer actively participating in politics, then that event is probably more history than contemporary politics (ex: "Why did Richard Nixon resign in 1974?").
However, if the question is about rules of political processes during historic events which still apply today, then it could be on-topic (ex: "Did Richard Nixon's resignation in 1974 create a precedence that an impeachment trial becomes impossible when the accused person resigns form their office?").
I don't think there is a cutoff date wise but rather what the question is asking. If it is asking more about events in general it might be history. However if it is asking more about the political side it might still be valid here.
The content should be more important than how long ago the content of the question occurred. It should also be remembered that it is possible for the question to be a good fit for both, one or neither sites and that isn't a bad thing. Though your question should only be posted on a single site even if it fits on multiple ones.