Ars has a great piece up about Android update times:
The winner for update times is, of course, the Nexus line. Stock software and a head start from being Google got KitKat out the door in just 14 days.
The bottom-line continues to be: If you care about Android updates, just buy a Nexus phone. Otherwise you’re looking at two major obstacles between you and a software update – the OEM and the carrier – and that’s assuming the OEM even cares enough to update their software. Carriers continue to have far too much influence on the phones people buy from them, and it’s hard for me to envision a scenario where I’d ever be happy buying an Android phone directly from a carrier ever again.