J.R.R. Tolkien spent about 12 years writing The Lord of the Rings before its publication began.
Tolkien began writing the sequel to The Hobbit in December 1937. The work developed slowly as he expanded its plot, languages, history, and geography. He completed the main manuscript in 1949, although revisions and preparation for publication continued afterward.
The first volume, The Fellowship of the Ring, appeared in July 1954, making the writing period roughly twelve and a half years from the start of composition to publication. The Two Towers followed later in 1954, and The Return of the King appeared in 1955.
The figure is approximate because Tolkien revised the manuscript repeatedly and publication involved further editorial and production work. It should not be confused with the decades he spent developing the broader mythology that eventually informed The Silmarillion and related writings.