I sometimes visit University Libraries, and have used several in the past for thesis research. Upon a recent visit to Portland State University Library, as I wandered through various subject areas and book stacks, I noticed that most of the books were 40+ years old.
That seemed strangely outdated. When writing my thesis in Grad School, my second draft, being a couple of years from the original, caused my advisor to suggested I insert fresher information/research sources.
Why then would a major university library maintain 40+ year old books? Of course some subjects, hard sciences like physics, chemistry, and or mathematics would change quickly. Literature, of course, contains many "classics" and would cover centuries. Social Sciences, would at first thought, have a slightly longer time frame than hard science to be considered out of date for research.
Is there any rule of thumb? Why yes, there seems to be.
I would guess that the main reason for not purging these old materials and replacing them with new, is funding. It would seem though, that current research materials would be a high priority for any major university library. The fact that many research items have gone "digital", may decrease the demand for material in "book" form.