Spock had just returned to the Enterprise, it seemed to me that Scotty was making a joke about not going back to Vulcan in a hurry.
He wasn't actually indicating that it would really take that long.
I never took that as any form of joke. Plus, given that Vulcan is 16 LY away from Earth and using the old TOS warp scale, it would have taken something like 11 days at Warp 8, or 15 at Warp 7. But as always, arguing speed WRT Trek is a pointless endeavor: the canon is hopelessly inconsistent and there is only one real speed: plot.