Yeah, I've always wanted to make my own movie as well. There must be hundreds of people like us. If we could all get together...
I don't know if you
could make movies totally cohesive to the book. You could get close, certainly, but at some point you have to realize that a movie is a movie, a book is a book, and so a movie will have its own rules. That said, you could be much more faithful to the books than New Line's production if you weren't catering to the plebeians (heck, you could be more faithful AND have better box office ratings, IMO).
But for your question, it depends on who makes the film. If it were me, and I did not care about what non-Tolkien fans thought, or about making money, then I might have to make them about 4 hours long, each, or more, to satisfy my own personal desire of completeness, without being ridiculous.
I think the best format would be a TV series - instead of movie-goers having to sit in their seats 4-5 hours, three times, to enjoy
Lord of the Rings: Unabridged, we could break up the story into 20 separate hours, thus having more screen time paced in an enjoyable fashion.
I know that didn't answer your question, the answer to which I couldn't even guess at.