What is the longest movie musical song?

According to IMDB, it's "The Continental," performed by Fred Astaire and Ginger Rogers in the 1934 film "Top Hat." It was surpassed in length in 1951 by the ballet in "An American in Paris," but the question asks for a song, not a musical number. "The Continental" had lyrics and was sung by the actors, so it fits the qualifications.

http://www.imdb.com/title/tt0025164/trivia

It is possible that there are some Bollywood musicals in Hindi and other languages with songs that go on longer, but I did not find them.

The movie wasn't "Top Hat", though, it was "The Gay Divorcee" (aka "The Gay Divorce"), as is shown by your link.