My own view (and I realize this is not shared by most) is that, if I have to "forget" before revisiting a book, most often it is a rather shallow book, relying on a "surprise" ending or elements which may not hold up that well, or contribute little new to it, upon rereading. There is, of course, nothing wrong with having such elements if they emerge naturally from the work itself (rather than being contrived in nature) and if they are, in hindsight, prepared for -- that is to say, the writer "plays fair" with the reader by laying the groundwork as things go along, even if it is done in such a way that the reader is highly unlikely to see where it is leading, until the final pieces are in place. If this is not the case, then I'm afraid i see such elements as meretricious rather than meritorious in any way; cheap gimmicks, in other words.
Obviously, this is not the case with someone like Hesse, who was a careful craftsman; so with his books, to follow up on the example, I wouldn't have to wait until I forgot anything because, even knowing all the details of the story, the next reading is likely to be a different experience, with different resonances and leaving a different impression. It is like a piece of poetry which is cyclical in nature, where the final stanza is a perfect (or near-perfect) repetition of the opening stanza, but because of what has lain between, it takes on a very different meaning to the reader, one which now casts its own light back upon the entirety of the poem. This, in turn, colors future readings of the poem, but does not necessarily contain them, for it may highlight other elements on other readings, which themselves alter the experience, sometimes quite considerably... and so on with numerous readings.
This is one of the reasons that I can go back to certain writers again and again over the years, often re-reading their works almost upon the heels of the previous reading, yet enjoying the work not as if it is new, or simply as one does an old friend (in the literary sense), but because the experience, the significance, is changed with each reading; one sees a different facet each time, and that facet is joined to all those one has been aware of before, adding to a greater whole. On the other hand, a lot of writers I may have enjoyed before don't hold up as well on this front and, while I would not hesitate to recommend them to someone looking for light entertainment or a way to pass the time pleasantly, I would hesitate to suggest them to someone looking for anything "meatier" in their diet.
Unfortunately, this also means that, having had such a heavy diet of this more substantial sort of writing, I tend to find the other more than a bit thin for my own taste these days, and therefore don't tend to read that much of it.
So the answer, for me, is that there is no "in general" here, really; it depends on the book or writer: is this work something which I can go back to frequently, without much of a pause between (whether I choose to do so or not is another matter), or is it something which relies too much on contrivances and gimmicks to "surprise" me, and therefore is much less capable of offering me a fulfilling experience under those circumstances. If the latter, then I may never reread it; or it may be years or decades before I have any desire to do so....
