To answer this most perplexing question, I watched the trailers of some of my favourite movies. Ben Hur is the greatest movie of all time. It was made in 1959 so if my theory is correct then the trailer should have been very short, and not have given too much of the film away. Fail. Four minutes! I know it's a 3 hour long movie but still...
Terminator 2 (1991) is unquestionably one of the greatest science fiction action movies ever made. The trailer is only 2 minutes long but still reveals too much of the film. Snippets of all the best bits. Well, I hear you say, isn't that what trailer is supposed to do? Whet your appetite for the main course? T2 had a teaser released before the trailer. The teaser ran for 1:21 and showed you nothing from the film. Brilliant! To me, that is what a trailer should be.
Parenthood (1989) likewise has a teaser which lasts 1:30. It shows you all the main characters, names the actors and has a scene of the family setting up for a photographic portrait which isn't in the movie. Perfect.
Bruce Almighty(2003) too long, The Matrix (1999) too long, Speed (1994), Scarface (1983) and Die Hard (1988) all too long. I watched these trailers and after 30 seconds, I thought, "That'll do! Don't tell me anymore, don't spoil it. I'm hooked already. When does it open in cinemas? Please don't ruin it for me with a turbo charged condensed version."
Why are they called trailers anyway,when they are shown before the feature film and are actually advertising for the film. I reckon they should be called teasers and they should be short. If you can't sell your film to me in under a minute, preferably in 30 seconds, then your film must suck.