How long was the Pony Express in operation?

Just 18 months, between April of 1860 and October of 1861. The Pony Express used relays of horse-mounted riders.

This express mail service was the US's East-West connection, operating between Missouri and California. The time taken for messages to travel this route was about 10 days. Despite operating for such a short time, the Pony Express became an icon of the Old West.

Both the first transcontinental telegraph (est. October 24, 1861) and the poor financial status of the Central Overland California and Pikes Peak Express Company were responsible for the service's demise.

https://en.wikipedia.org/wiki/Pony_Express