I think at least 100 years it could be enough time for the Canon Dovahkiin to die of Old age.

If it is set in a different province, why does it matter if the Dragonborn in Skyrim (the province) is still alive or not? As long as they keep details/rumors vague and ambiguous, it shouldn't matter, IMO.

The Nerevarine could have still been alive in the events of Oblivion, I think the developers handled it well enough.

Cant really say. 100 years, 1 day...who knows

But really, Morrowind-Oblivion was 7 years so Skyrim-TESVI can be 10 years for all we know.

Whatever the case it'll probably be long enough for the outcome of the civil war to have become insignificant, so that they don't have to specify what's canon. Unless people just don't mention it at all or in vague 'the war is over' terms.

Let's not rule out the idea of a sequel taking place in the past, simultaneously with Skyrim, or during a Dragon Break