We know that the keyblade war happened long enough to be a children's story, falling into legend, and we also know it was definitely long enough for the average person to have pretty much no idea what it is or what a keyblade is. Now, the universe was rebooted, of course, but if everything was changed to normal except divided across the universe, people should still know about the keyblade, and, while very few people do, we know that, because of Luxu, knowledge of the keyblade war survived.
Now, look into the present-ish day and we see that Xehanort was interested about the keyblade war since he was very young. We have some of this confirmed and some of this revealed to us through these reports, but here are some key points he drops on us:
1. Xehanort describes that the combining of Ventus and Vanitas will create the "stuff of legends", meaning that this world, indeed, must happen, at the very least, centuries after the old world (maybe only a couple hundred years) since it is but a mere legend from the present-ish day.
2. When Xehanort mentions the X-Blade, he has to infer the causes of such a war, (sort of) incorrectly coming to the conclusion that the entire war was over the entity Kingdom Hearts. This could indicate that the true story of Back Cover may have never been passed on or has simply been lost in the sands of time.
3. Xehanort notes in his reports that he knows of precious few keyblade wielders outside of his circle, and that he believes that more may be out there. From this we might be able to infer that the keyblade war happened centuries ago, but perhaps not too long ago due to the small number of wielders existent in the realm of light (might be more populous if they were around longer).
4. The keyblade war is a known fact while the details of said fact are a fairytale to the common people. This might not indicate a specific time period, but we know for a fact that Luxu's apprentices and the Dandelions kept the knowledge going throughout the ages.
5. While this doesn't indicate a specific time either, Xehanort has suspicions of his master and Eraqus when he is told that the origin of the keyblades was to protect the light. He found this answer unsatisfying, and thus he went on to do further research on the keyblade war and the age of fairytales. If this information is hard to find, we can infer that the Dandelions, and even Luxu's apprentices, wanted to keep knowledge of the war out of sight, meaning we might never have a clear answer of how long ago this happened.
From this information, we can't infer a specific time as to how many years stretch in between Birth by Sleep and Unchained X. However, we can infer that Xehanort certainly isn't the first apprentice of Luxu's line, and that it seems to have been going on long enough that information has been largely fitted to the keyblade wielders' narrative,
and that outside knowledge of the keyblade war has been restrictive and largely been kept under lock-and-key for a long time, meaning this would only spark Xehanort's curiosity further. Thus, we can infer, at the very least, a couple of centuries have passed since this time, due to information being limited and largely forgotten about.
Sorry for the unclear answer, but I hope the Xehanort reports helped a little