The "timberline" is so named because this is about the heighth that the forests end and the open snow and rocks begin.
The Timberline trail circles Mt Hood, and is approximately 25 miles around the mountain.
Hikers usually break up the complete jaunt into two or three nights, putting in about twelve miles a day.
Day hikers do smaller loops over sections of the trail, driving to reach the trailheads of the furthest sections.
Often in spring, river crossings are completely impassable due to glacier runoff.
The complete route can usually be done mid-summer when the rivers are lowest.
I've done a twelve mile loop in one day, but this only incorporated 5 miles of the Timberline trail itself, and the rest was to and from.