There is no time frame, no criteria, and there should not be. Something becomes classic because it leaves a lasting impact on what follows it. Example The Clash's self titled cd is a classic because it became an archetype for a sound. In 5 years if we look back and realize that commons BE became an archetype for the sound of hiphop of the current time then I will say its a classic. I'm not nessiciarly saying that we'll know in 5 years or 10 years but we'll know in the future.