0
If you count The Decalogue as one movie, then that's it for me. It's 9 hours and 32 minutes long. But since it's really a miniseries that just happened to receive theatrical showings in several countries (including the U.S.), then I think the longest movie not intended as a miniseries that I've seen is probably Kenneth Branagh's Hamlet, which is 4 hours and 2 minutes long. The full-length version of Fanny and Alexander is longer than that at 5 hours and 12 minutes, but again, that was originally a TV miniseries. So I think it's Hamlet.
To the OP, did you watch the theatrical cut of The Last Emperor or the TV version? Because if you watched the theatrical cut, the TV version is just about an hour longer. Also, if you watched the theatrical cut and that's the longest movie you've ever seen, that means you've never seen The Godfather, Lawrence of Arabia, Seven Samurai, or any other movies that are longer than 160 minutes, in which case I should offer you some must-see recommendations, including the three I just mentioned.