What is the longest time for a movie being in theaters?

Longest First-Run of a Film in One Cinema

Romance in Lushan (China, 1980) first opened at the Jiangxi Movie Circulation and Screening Company in Lushan, China, on July 12, 1980, and has be shown daily four times a day since then.

According to Guinness World Records, this film has the Longest First-Run of a Film in One Cinema. It premiered at the Jiangxi Movie Circulation and Screening Company in Lushan, China, on July 12, 1980 and has been shown daily up to now. Watching the film at the said cinema has become a tourist attraction.

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

Across Multiple Cinemas

Rocky Horror Picture Show

This is a little harder to prove because of the "different cinemas" part, but this is likely the best candidate.

The Rocky Horror Picture Show, which is still being shown somewhere virtually every week, 38 years after its initial release.

Still in limited release four decades after its premiere, it is the longest-running theatrical release in film history. Today, the film has a large international following. It was selected for preservation in the United States National Film Registry by the Library of Congress in 2005.

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