The length of a "Derby" race depends on which Derby you are referring to:

• Kentucky Derby (USA): The Kentucky Derby is run over a distance of 1¼ miles (10 furlongs or approximately 2,012 meters) at Churchill Downs in Louisville, Kentucky. The race typically lasts around two minutes, earning it the nickname "The Most Exciting Two Minutes in Sports." The race was originally 1½ miles but was shortened to 1¼ miles in 1896 to better suit three-year-old horses

• Epsom Derby (UK): The Epsom Derby, also known simply as "The Derby," is run over a distance of about 1½ miles (1 mile, 4 furlongs, and 10 yards, or approximately 2,419 meters) at Epsom Downs Racecourse in Surrey, England. This race is one of Britain's most prestigious and oldest horse races, dating back to 1780

In summary:

Derby| Distance| Approximate Length in Miles| Location
---|---|---|---
Kentucky Derby| 10 furlongs| 1¼ miles| Louisville, Kentucky, USA
Epsom Derby| 1 mile 4 furlongs 10 yards| About 1½ miles| Epsom, Surrey, England

The Kentucky Derby is shorter and lasts about two minutes, while the Epsom Derby is longer at about 1½ miles