IMHO, to call a book, movie, or other piece of art 'classic' is a combination of critics' praise and audience staying power. If something becomes a vital part of national or international dialogue (people's lives), it is a classic. There is certainly subjectivity involved: to keep it in the realm of our host's books, compare
The Stand and
Lisey's Story. Because it has remained vital in public conversation and influence (who here can't off hand name several movies, books, even songs, that have drawn influence from
The Stand?), and critics have grudgingly have come to admit that it has merit, it is a classic.
Lisey's Story is objectively a more accomplished work, by far Mr. King's most 'literate' book, and got positive critical attention from the start, but it hasn't captured the public's affection (yet), so it isn't a classic. Fits one criteria (critics), but falls short in the other (public opinion). Something that gets just one or the other (critics or public) usually won't be considered a classic. Sort of weird, but there it is