Is there a definition of history separate from the definition of politics?

I think that what you're attempting to find an answer for is the basis of one of the ideas introduced by James Loewen in his book Lies My Teacher Told Me. In it, he distinguishes between what he calls "sasha" history (history which is still remembered by people who are alive toay) and "zamani" history (history which is now, essentially, dead and academic). World War II and the Hiroshima bombing were in the sasha realm as recently as the mid-90s when the Smithsonian Institute wished to put up a rather British style display on the subject (by which I mean even-handed and not flattering to the home country) but was denied in favor of a more Bowdlerized version of the events. Likewise, American slavery is clearly in the zamani zone of history but the legacy of racism and inequality which stem from it is clearly not.

That being said, there is no real dividing line between the two subjects, even though they do cover different things. History gets politicized all the time, but so too does politics learn from and adapt to the lessons of history.

To take the particular case of Kosovo, it's pretty recent history and so very clearly in the sasha range. You don't even have to be all that old to remember the conflicts in the former Yugoslavia in the 90s or for that matter the tribunals which were held in the early 2000s (the first of their kind since the 40s, if memory serves). It is unsurprising that people feel this is politics at this point; a great parallel with some of that situation are the Nuremberg Trials, about which a teleplay was written and shown in the late 1950s in the US but which was very heavily Bowdlerized due to the politics of the time (for example, Westinghouse demanded that the creators excise any reference to "gassing" lest the people of America associate gas ovens with Zyklon-B and Auschwitz). Ironically, this politicization of what is now a purely historical event is itself history. There's some kind of Inception joke here which I am sure could be made by a funnier person.

So long story short, there's not a real dividing line but that doesn't mean a distinction does not exist. 20 years is clearly too soon to separate politics from history. 50 years still seems pretty soon nowadays, depending on the event we're talking about: think of the JFK assassination as an example on one side (an event remembered by schoolchildren, many of whom are only in their 50s and 60s now) and the Bay of Pigs invasion on the other (something which was pretty big news to adults, but that cohort is now in their 70s and 80s and beginning to die off). World War II itself is probably pretty firmly into zamani territory by now, although it's interesting to not (as above) that as recently as 20 years ago it was not. World War I is without question zamani history.

You will not get anywhere by trying to divide events as historical or political. The same thing can be both, and it depends on how you approach it. History is any event that is approached with a historical mindset - using the tools of historical research. A political scientist or political commentator could approach the same events as political events. The distinction is not in the event.

Therefore, I don't think you can set any cutoff point to determine if something is an historical event or not. Even a current event happening live - like, perhaps, Russia annexing the Crimea - is a historical event. Good commentary on it would involve a political commentator to give a political explanation of what is happening, but also a historian to give the historical perspective. Events do not occur in a vacuum, and there's always an historical context.

The same can be applied to any relevant discipline. A movie comes out. Is this a cultural event? An historical event? An entertainment-history event? The answer is, of course, all of the above, depends on how you interpret it.