As I continue scrawling page after page of my first book, I can't help but think that I have enough material planned for four or maybe even five books. The reason it stretches beyond a trilogy, with one book for each generation, is that there are significant events that transpire which will fit well as the climax for each tale.
Thinking about this leads me to draw comparisons with other movies and books.
Forrest Gump - the run time of the movie is 2 hours and 22 minutes, but covers Forrest's life from the ages of around 7 to 38. The movie itself isn't what I would consider overly long, but it feels so by the end due to how much time is covered within the story.
Legends of the Fall - 2 hours and 13 minutes isn't long for the 60+ years that are covered in the telling.
Lord of the Rings - The Hobbit and the trilogy cover some
Bilbo is 50 when Gandalf draws him into the quest with the dwarves, during which he finds the One Ring. He was 131 when he boarded the ship for the Grey Havens. Frodo was 33 when Bilbo left him the ring after their shared birthday (that's right, both were born Sept 22) party. He was 53 when he accompanied Bilbo to the Grey Havens. The main line of the quest to destroy the ring is one month short of a year. Yes, I know, the movies and the books differ. In the book it takes 17 years for the Ring to leave the Shire, that's a long time to hold onto a really crappy birthday present!
The Wheel of Time series covers ages of repeating history, but only follows the main character of Rand al'Thor from the age of 20 to around 22.
And last, but by far not the least, the Time of Legends trilogy follows the hero Sigmar from the eve prior to his first battle to his abdication, ages 15 to 80.
I am by no means comparing my as-yet-to-be-finished, first book to these favorites of mine. I am merely illustrating that great tales can be told in a few hours to thousands of pages.