It took the Corps of Discovery a year, six months, and one day. This translates to approximately a year and a half.
The Lewis and Clark expedition set out from St. Louis, Missouri on May 14, 1804, and arrived in proximity to the Columbia River on November 15, 1805. They came back to St. Louis in late September of 1806, where all members of the expedition were rewarded double pay and hundreds of acres of land each. That was a lot of rewards allotted, given there were 45 men who departed!