Filmmaking / Directing : How long is too long by Bert Newsom

That's a loaded statement, but coming up with an answer, that really depends on the story, the business plan, existing commitments to your project, and your personal drive to get it in front of people, ETC! If you have already qualified the project as having a solid stance in the market and that it could/should (obviously, never promoted as a guarantee unless you want to die early) provide a return to investors, then the challenges should be minimized.