Most of the time, and for most people, “forever” is that piece of time that we can see with our own eyes. Forever is the length of a single, human life.
This, I believe, is why a book feels permanent, even though enough libraries have burned over the centuries that we ought to know better. A well-made book, stored upright, in a dry, dark place, will survive a hundred years—that is, a lifetime. More if it is especially well printed, and only carefully handled, but a hundred years is a safe bet. Plenty of time to read it as a child, hold onto it through adolescence and adulthood, and then give it to your first great-grandchild. That’s as much forever as any of us can reasonably conceive.