Those date due cards have fewer than 20 lines, if I recall correctly. Either way, it really depends on the book. People can put library books through the ringer sometimes, but books are pretty durable. I'd say our collection loses less than 5% of its items to damage. We discard things for the more nebulous "condition" more often, but generally that's paired with some other criteria like not circulating or being out of date.

Hm, I worked at a national library during my university years, for some extra cash and free access to the more silent part of the building.

State/National libraries in Europe often work on a "required (free) copy" aspect: publishers are required to provide the national library with a free copy of any book they publish. This makes them huge, but it also makes them have all kinds of books of different quality, both content-wise and built-quality-wise.

Most books are never checked out. Those that are get checked out rarely (unless the book is incredibly influential or some professor makes them required reading for their course). 30-100 checkouts may translate to 20-60 years of being in library circulation. But those are mostly spent sitting on a dry shelf, when no damage to the book occurs during normal operation (e.g. no fire / no water damage).

30-100 checkouts, each for two weeks (with the option to extend another two week) spells 1-6 years the book will live with other people - and here is where things happen. Books are transported FROM the library to houses and flats (often in containers that are not very good for the purpose, think backpacks, canvas bags - often leading to shear to the pages and stress on the binding). They are put in environmental conditions that differ vastly (extreme cold in winter. High humidity). Individual pages are being turned and worked on, creating stress on the binding. People have coffee cups that fall, and pet cats that urinate. Pages sometimes rip. People also have kids with crayons. They themselves have text markers and pencils. They may overstretch the binding, to "make the book lay flat", or read in bed, or in the bathtub. And eventually, they bring the book back.

Of course, the lady who takes the book in will do a quick check on whether the book is in an acceptable state - but she has no idea how it looked when we lent it out. In the bad cases (again: cats urinate), the book is immediately discarded and replaced with a new copy if possible. For less bad cases, damage still creeps in and accumulates. And eventually, glue will give in, pages will become loose, the book will fall apart.

Compare that to the privately-owned book, which is often bought, read once or twice, and lives on a shelf for the rest of it's existence. That one also gets damages, but they don't count up that much.

National and University libraries often have contract or in-house bookbinders who will repair a damaged book, which means removing the cover, and rebinding the book into a "library binding" - that often comes with a more sturdy cover or - with softcover books - a thick plastic film over the cover. Many libraries also clip the edges of softcover book to make them rounder, which increases longevity. But ultimately, there is no more thing to fix.

I think 30 checkouts is a good estimate for most softcover books in public libraries, 100 being on the high-end, high-maintenance, highly-invested national library end.

I think this context is important, thank you for providing it.

If i were to summarize succinctly I would say the lifetime of a book in a library is akin to the lifetime of a car at a rental car company. The natural wear and tear on it is far greater than one that is owned by a single, private owner, and so the expected lifetime becomes much shorter.

Great comparison. Individuals treat their possessions much better than they treat others.