I got this question recently:
“My screenplay is LONG (175 pages), which is viable for a 3-hour movie, but I’ve heard that producers and contests won’t read anything over 120 pages, which is odd because the majority of the most successful movies ever are 3 hours long (Godfather, Dances with Wolves, Titanic, Avatar, Braveheart, etc.). What are your thoughts on this issue? Can I do anything proactive to get people to read this script?”
Scott over at Go Into the Story wrote a good post about this recently. Check it out. Look at the comments, too. Lots of good opinions. Bottom line is that you’re correct that some great movies have been really long but every great movie you mentioned came from an established pro. Making a 3 hour epic isn’t something that beginners can usually pull off which is why writing a really long spec isn’t usually recommended.
Here’s the post over at Go Into the Story: http://www.gointothestory.com/2010/01/question-what-about-presenting-187-page.html