There is something magical in the last 100 pages of a book. This is particularly the case with fiction. In my experience this magic holds true for everything from lap breakers, like World Without End at 1024 pages, to slim young adult novels like Amanda Hocking’s Hollowmen, a very fast 194 pages.
The last 100 should go faster than the rest of the book and not because the first part was interminable. For me, delivering in the last 100 pages forgives a multitude of sins.
100 pages seems to be right length to build up an ending without rushing it too much. When there are 15 pages left in the book and I’m worried that the author can’t wrap things up in time it usually means the ending is rushed and I’m not going to be happy. I want some build up. I want to see the ending coming even if don’t know what the end will be.
Spend a lot of time on those last 100 pages. They are the last thing we read and the last thing we remember.