Fourth, Fifth, Sixth (and Seventh?) Ages

[Arvegil145]

This might be a good place to post this quote:

Of Eldarion son of Elessar it was foretold that he should rule a great realm, and that it should endure for a hundred generations of Men after him, that is until a new age brought in again new things; and from him should come the kings of many realms in long days after. But if this foretelling spoke truly, none now can say, for Gondor and Arnor are no more; and even the chronicles of the House of Elessar and all their deeds and glory are lost.

- The Peoples of Middle-earth, 'The Tale of Years of the Third Age', pp. 244-5

So, if we take a normal human 'generation' to mean something like, say, 25 years - that would equate to c. 2,500 years.

And if we take Eldarion's death to be c. FO 200, that means that the Fourth Age lasted for about 2,700 years - which checks out with Tolkien's hastening Ages.