I've wondered the same thing. I think it will change. It's hard to imagine though, because the rate of consumption has sped up so much. Now, songs from two summers ago feel old and "classic" feels like this static thing of the past. It has to change though, but I just wonder what will be considered classic and what won't age well.
Burial - Burial
Dilla - Donuts
TV on the Radio - Return to Cookie Mountain
Cat Power - The Greatest
Darondo - Let My People Go
Kid Koala - Your Mom's Favorite DJ
Amy Winehouse - Back to Black
The Killer's - Sam's Town
Alexisonfire - Crisis
Quadra - Based on a True Story
Madvillainy is probably my favorite, but The Mouse and the Mask as well as Mm... Food are close behind. Still thankful for his feature on Gorillaz' Demon Days project, b/c that was my first introduction to the masked MC.