If you add all footage from the trailers, it can be about 10 minutes, maybe more.
I enjoy movies orders of magnitude more now not already having seen the set piece scenes / heard the important lines.
You could presumably then tailor trailers to particular groups to emphasise desired attributes.
Downside to that is that if you sit through a few trailers you could literally have seen a-third of the movie before you go to watch it.
If you have seen the movie True Grit - either the old or the new one - compare the trailer to the 1969 version, which essentially gives out all the plotpoints - http://www.youtube.com/watch?v=tN-j4GDqjv4 - and the trailer to the 2010 version - http://www.youtube.com/watch?v=CUiCu-zuAgM .
Note, I don't go to movies because I normally leave disappointed.
It has been proven that once people have an inferior version of something they are more willing to pay full price for a good version.