Someone on facebook, a former grad student in my department who is now editing the book review section for a journal, suggested that it takes 72 hours of work to produce a book review of 800 words or so. The context is that publishers now want to send ebooks instead of hard copies of the book to the reviewers.
Now it takes me much less time I read about 60 pages a minute, so that would be 5 hours for a 300 page book. Then I would write the review in about two hours, spend another half an hour on another day and tweak it. I can write a paragraph of 250 words in less than an hour, etc... I don't think I've ever spent more than 10 hours on a review, and I think my reviews are well-written and responsible. The editors rarely ask for changes.
I would give similar answers to doing a peer review of an article. I would read it in an hour, then the next day spend another hour writing the review. As I'm reading something, I start to form opinions and draw conclusions, and then I start to formulate sentences in my head. I make a note of page numbers I want to come back to to cite, etc...
I'm sure the 72 hours is reflective of someone's experience doing this, but then I wonder if hypothetical-person-for-whom-it-takes-this-long should be even doing book reviews at all. That would be basically all the person's research time for two months. If person is grad student or non-tenured, then he or she needs to allocate energy correctly. If said person is that slow in doing things, then it is hard to see how they could write a dissertation or an article, so doing extra tasks that are extremely time-consuming is not well-advised.
I do not mean to be criticizing my former student, for this has opened up for me a perspective that is valuable to me: I might be faster than other people doing the same amount of work. I don't cut corners, but I work efficiently on things like this. I tend to prioritize things that are significant, so in a tenure review I won't write summaries of someone's arguments in all their articles and books, but say in a few words what the strong points are. I have to be careful, because a very short letter would look bad, even if I can say all I need to say in a short space. I typically have to flesh things out more just to make the letter longer and more substantial in appearance. As a reader of such letters I typically find that 3 pages is ideal. Four or more pages of a tenure letter means that the writer is doing a lot of summary or entering into non-essential detail. One or two pages is not enough for a substantive review.
Once again, I would typically read a page a minute and mark things I want to quote, then take a few hours to compose the letter itself, maybe an hour a page at most.