[pseudohadamard]

The director's cut of Apocalypse When at, I dunno, about nine or ten hours (take a blanket and pee bottle), is definitely worthwhile, but most films that have gone over about two hours seem to be just the director being so impressed with their own vision that they didn't know when to stop. But then it's also quite a balancing act, there are some films where the studio insisted on messing it up (Blade Runner is a prime example) and others where the studio should have reined in the director a lot more than they did (many films where the director had a huge hit with film n and was given free rein to do whatever they wanted in film n+1).

It doesn't even have to be too long to go off the rails, for example Stalker, if you haven't read Roadside Picnic, is just two nutters wandering aimlessly around the landscape for 2+ hours (Tarkovsky allegedly said that it needs to be slow and boring at the start so that people who walked into the wrong film showing had a chance to leave), and if you have read the book is nothing like the book. It's a fabulous film but presumably completely baffling to anyone who hasn't read the book, and also somewhat unsatisfying to someone who has.