There is a Russian animated film that has been in production for over 30 years.

Called The Overcoat, the film has been the project of acclaimed Russian director and animator Yuriy Norshteyn since 1981. It’s been in development for a long time, and, as of 2004, only 25 minutes of the film had been completed!

A number of reasons are behind the lengthy development time, but they are mostly budgetary and circumstantial. By 1986, Norshteyn had only completed 10 minutes of his film; mostly because of how preoccupied he was with other projects he was working on. After being fired from the animation studio he worked at, he and his team set up their own studio, but because funding was sparse, work was still slow.

From 1999 to 2001, production stopped outright because of the death of Norshteyn's close friend, and the cinematographer of the film. Production was stopped for another year while he worked on his book, Snow on the Grass. As of 2011, however, the film has completed filming and is finally in postproduction.

(Source)