How long was Tristan Ludlow away from home?

The 1994 film Legends of the Fall describes the film's main character, Tristan Ludlow being away from home for a lengthy period of time after he decides to leave the ranch and his lover, Susannah Fincannon in Montana in search of himself after witnessing the death of his brother in WWI.

Susannah first comes to the ranch during the summer of 1913. When Susannah meets Isabel Two, she tells Susannah that she is 13 years old. Considering the dates of the letters that are self-narrated by the authors, Tristan had departed from home some time around 1918. Tristan sends a letter to Susannah breaking off their love affair in 1919. The film's narrator then goes on to describe Tristan being gone for "many years." Upon Tristan's return, (presumably 1920) he marries Isabel Two and she is 20 years old. Decker also explains to Tristan (on the day that he returns) that Susannah and Alfred were married "several years ago." Susannah was still at the ranch awaiting Tristan's return in 1919.

1913

Samuel Ludlow and fiance Susanna Fincannon arrive this summer for Susanna to meet Samuel's brothers and father. (Ludlow Homestead, Montana)

William Ludlow laments his inability to properly raise his sons, saying he is better suited to leading soldiers. (Ludlow Homestead, Montana)

1914

Alfred, Tristan and Samuel Ludlow leave home for Canada to join the war in Europe "to defend an England they've never seen." (Rocky Mountains of Montana, USA)

1915

Samuel Ludlow writes a letter to his fiancee Susannah describing his experiences in the war.

Tristan Ludlow writes his father and Susannah about how he is being discharged from the British Army and how Alfred is bringing Samuel's heart home.

Alfred Ludlow writes to his mother about how he has found his place in the world describing his business opportunities in Helena, Montana. (Helena, Montana)

1918

Susannah Fincannon writes to Tristan asking if he will ever return and about Alfred's successful business ventures.

1919

Tristan writes to Susannah telling her how he has become a hunter and that she should marry another man.

As narrated by One-Stab:

After that, Tristan sent no more letters. As the years passed by we would hear that someone had seen him on a ship going up some river no white man had gone up before. Stories came to us. Strange stories And then, for years, there was nothing. He was lost to us. That was all we knew. But every year, in the Moon of the Falling Leaves, I would dream that the bear's voice inside him had grown silent and that Tristan might again come to live in the world. But then the winter would come And then another spring. And still he stayed away.

Is there a reason that the filmmakers chose to embellish the length of time that Tristan was actually away from home? Is there any evidence (besides the age of Isabel Two) that puts Tristan back home at the ranch in 1920?