I’m not sure of the real expectations; only Blizz would tell us but multiple sources suggest +/- 2 years for this classic+ content
Dungeon reworks. New Ashenvale PvP things. Could be wildly successful. Worth its own realms on the forums.
With every leveling phase lasting about 2 months and the regular vanilla phases around 2 months as well plus the extra raids they planning post naxx, it’s safe to say that this season will last longer than 2 years