The "classic" nature of a book is only measured by the time it's been around, how long it's survived. So, let's take a popular book, Harry Potter for instance. For it to be considered a classic (to me, it still isn't), it has to be around for at least some decenia. This, of course, doesn't mean a book is bad or inferior in any way. Just not that resilient to the sands of time, if that makes sense (Not a native speaker, people! ). A classic is a work people keep coming back to, that's universal, and applicable to almost every age in time.