Most presses will have me index while the author completes final proofreading. The time allowed for this is usually three weeks to a month.
In terms of parts of the book indexed, I will automatically look for indexable material in the foreword, preface, introduction, conclusion, illustrations, appendices, and figurative matter, as well as the body of the text. I generally do NOT index the acknowledgments and the references or bibliography. Notes may or may not be indexed. Current practice is to pick up material from the notes only if they contain relevant and substantive information about topics not accessible through the text. Names of secondary references (modern scholars) are indexed only if the text directly and specifically discusses their work or theories in some detail. Clients who want to include material in the notes, or a thorough list of all referenced authors, should make sure to discuss this specifically with me.
Next, go to the index and pick a term from the first column. Look at the pages it references. Does the term relate to what’s on the page? Are the page numbers accurate? Check at least one entry from every column in the index. Even a really good index might contain a few errors; but you shouldn’t find many. If the index doesn’t seem to be working using these techniques, or if you find more than three errors, it’s time to talk to the indexer about what seems to be the problem. A good indexer will be happy to work with you to make things right.