Cerca de 6000 anos atrás. Mas eu não acho que os detalhes exatos de quando eram importantes para Tolkien.
Tolkien had written multiple times that Middle-earth is located on our Earth. He has described it as an imaginary period in earth's past, not only in The Lord of the Rings (see Prologue and Appendices), but also in several correspondence letters, estimating the end of the Third Age to about 6,000 years before his own time, and in N.W. Europe (Hobbiton for example was set in same latitude as Oxford), though at times he would also describe elements of the stories as a kind of "...secondary or sub-creational reality" or "Secondary belief" in replies to letters.