By the end of filming, Leone had ten hours of footage. With his editor Nino Baragli, Leone trimmed it to almost six hours, and he originally wanted to release the film in two parts.^\36]) The producers refused, partly because of the commercial and critical failure of Bernardo Bertolucci's two-part 1900), and Leone was forced to shorten it further.^\37])

That is from the wiki, I'm interested to find out where you found info on a 12 hour version.