I suspect it varies between publishers, but is it known to what extent Google limits page views in Google Books? Rough estimates are fine.
Publishers upload books via the Google Books Partner Program (http://books.google.com/googlebooks/tour/). A publisher sets the fraction of a book that can be previewed. This is how the help page describes it:
You have control over the percentage of pages that a user will be able to view during a 30 day period. This value can be set between 20% and 100%. Partners can also choose to make their book 100% browsable with the option to download a PDF.