Yeah, but then you're choosing a story to fit the run length rather than choosing a run length to fit your story. You'll always limitations regarding resources and time, but in the advent of digital photography, you can record days of footage with a few cell phones and put it all on a few of portable USB hard drives, so the cost of actual film is basically nonexistent. Limiting run length is entirely arbitrary, especially if you're a computer graphics guy and you can just film people against a green screen and do all the sets and backgrounds as CGI in post production on your own personal time.
No, wait, you can't do that, because if you do CGI for a living, you're a "professional", and that would make the Spirit of Fan Films(TM) very sad.