Which rover's mission was planned for 90 days but lasted over 14 years?

The rover whose mission was planned for 90 days but lasted over 14 years was Opportunity. NASA's rover landed on Mars on January 25, 2004, and remained operational until its final communication on June 10, 2018.

The original 90-sol plan was intentionally conservative. A sol is a Martian day, slightly longer than an Earth day, so the planned mission was about 92.5 Earth days. Opportunity ultimately operated for 5,352 sols, roughly 55 times its designed surface lifetime, before a planet-wide dust storm cut off its solar power.

Opportunity was built to investigate whether liquid water had ever existed on Mars. It found geological evidence of ancient watery environments, examined rocks and soil, and traveled about 45.16 kilometers across Meridiani Planum—far beyond its original driving target.

The rover is often confused with its twin, Spirit. Both were part of NASA's Mars Exploration Rover mission and shared the 90-sol design goal, but Opportunity lasted much longer. Spirit became stuck in soft soil and stopped communicating in 2010.