Back in the 1930s, the editor Geoffrey Grigson described the particular thrill of receiving a handwritten manuscript from the young poet W. H. Auden and typing up the lines before sending the new poem off to the printers. He compared this now obsolete procedure to the development of a photographic negative (a process now equally obsolete), and liked to see that moment as the point at which a new poem entered the culture.
But when does a poem actually become part of the culture? When it's first written? When it's accepted for publication? When it first appears in a little magazine or anthology? When it has a broader public exposure by - for example - being recited in public or on the radio or quoted in a film? Or is it when (to personalise the matter) one reads it for the first time? When a line, or lines, become fixed in the memory? When it's quoted elsewhere, or borrowed by another writer?
Fiction is not like poetry because novels are not like poems, and novels are not (yet) as remote from the cultural mainstream as poetry. Of course a similar if more technology-driven route is taken from manuscript to publication, but poems are slighter, lighter, less emphatic productions, less commodified. This is not to question their power or value or vestigial prestige, only their presence.
So - to reformulate the question - when does a novel become part of literature? Answers on a postcard, as we no longer say.