The longest movie ever made was a 2012 Swedish movie called "logistics" which follows the production of a pedometer in real time but also in reverse, the movie is 51,420 minutes (857 hours or 35 days and 17 hours).
The longest proper cinematic film is black and white 2019 Bangladeshi film called Amra Ekta Cinema Banabo (We Will Make A Film in english, the movie was also called "Innocence" in promotional material.) and has a run time of 1265 min (21 hours and 5 minutes). The plot is a little convoluted, but its basically a drama that takes place in the aftermath of the Bangladesh Liberation War.
Here is a brief plot synopsis for anyone who is curious: https://filmfreeway.com/AmraEktaCinemaBanaboTheInnocence-21hrs