Hollywood movie that took longest to create

If the John Carter (2012) answer from Dr R Dizzle isn't what you are looking for this it's this:

It's a box office & Yahoo offer the answer.

The Thief and the Cobbler

The animated movie The Thief And The Cobbler holds the record for a movie to be in production for the longest time. It was in production stage for 31 years. (1964-1995) Then there's Love & God (1986) directed by K.Asif. He started the film in color after Mughal-E-Azam casting Guru Dutt & Nimmi. Guru Dutt died in 1964, shooting came to a halt. Then, Sanjeev Kumar was taken in the place of Dutt, but Asif himself died in 1971. The film was left incomplete. Later in 1986, Asif's wife revived the film & released it in the incomplete form. Sadly, one year before it's release, even Sanjeev Kumar died.

Although this might not be exactly what you are looking for, John Carter (2012) first entered production in 1935 as a hand drawn animated film, which means John Carter was in production for nearly 80 years before it was eventually released.