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.