It is different for my hobby areas, where a lot of the textbooks are history, archaeology, and anthropology-related and relatively short (200-300 pages). The fact that there are occasional 500-600 page textbooks that take me a week or two are not a problem.
Hmmm on reviewing the books I have read, it seems my original list included some things that should not have been there. A quick assessment of ratios suggests I should drop it to 70-75 per year.
I can't speak for the person you're responding to but for me; ~400 pages of content, if it's dense I assume a pace of about 30 pages per hour, so about 14 hours of reading, maybe three hours of review total on top of that. If you're learning math concepts invest an hour or two of practice for every hour read.
If it's more light reading or less foundational I can do a bit less than page a minute. This week I trudged through EffectiveUI pretty mindlessly (a lot of O'Reilly books are very wordy and soft) in about six hours.
