Typically a thru-hike is completed in one year. How a person breaks it up is their decision.

What constitutes a thru-hike is a constant source of discussion on White Blaze. As long as a person isn't trying to set a record, I say a thru-hike is whatever a person says it is.

For myself, I lean towards the White Blaze mentality. For myself, I wouldn't call it a thru-hike if I had to miss a section due to a hurricane, forest fire, or other cause. This is an extreme view, but I feel luck should play a role. If others want to skip sections and call it a thru-hike that's up to them. People often do that on the Colorado Trail because of snow conditions.

Usually people who spread a trail out over years are called segment hikers.

In my opinion, the adventure is what matters and not some arbitrary concept of a thru-hike. I've read many books on the Appalachian Trail, and some of the best books are about people who didn't complete the trail.

(I'm practicing to be a politician, so I took both sides. Later, I'll deny I ever took the less popular view.)