David Smith writes about iOS adoption rates. It seems that it just takes over a week to get to 50% adoption whenever Apple releases a new version of iOS. The second chart says it all: 10% is still on iOS5 (and even 4.x) but the rest is on iOS6 or greater. 70% is on iOS 6.1.3.
That's pretty impressive.
(Also: I love how the graphs are interactive)