From experience I would suggest as much as possible. Many writers/editors try to be smart and reduce the size of the index, but then you end up looking for something you know is in the book, but you can't find it. If you want include a qucik index with the most common or prominent items too.
The other thing is the TOC, I would suggest two versions there two, a quick TOC on 2 pages max, where you just see the main headers. And a little longer one where you can see everything in detail.
Nothing is worse than having a good book that you can't use properly, and people are used to full text search from almost every computer program, so you should have at least the possible paper equivalent of that.