Well, at least on HB summits, the longevity of these books seems to be limited mostly by moisture and rodent action! The 1987 summit book on Gulderturm has a note on the first page saying that “the old book has been nibbled by a rodent” 
Apparently the summit book on HB0/LI-001 has suffered the same fate. Here is the page where the first activator @PC5A signed it:
Usually they’re placed in sturdy metal containers though.
Finally, the oldest summit book that I’ve found so far was from 1970 on this summit (photos of the book in the post):
To me, it’s a magic moment when I arrive on a rarely climbed summit, with only a small cairn as a sign of human activity, and find a summit book that has been up there for decades.