How long did it take for Lewis and Clark to explore the Louisiana territory?

Before Lewis met up with Clark, he began the expedition on August 30, 1803 in Pittsburgh PA. Lt. William Clark would offer to join Lewis on the expedition weeks later on October 13, 1803 at Camp Dubois (in present-day Indiana).

In spring 1805, they continued to the headwaters of the Missouri River, struggled across the Continental Divide, and headed west along the Salmon, Snake, and Columbia Rivers to the Pacific. The expedition then ended on September 23, 1806 upon their return to St. Louis from Oregon. So the expedition took about 3 years.