On average, how fast do iOS users upgrade to the latest version?
Until recently, I have been happily ignoring iOS5 SDK because its new features aren't really essential to the products I'm working on. Also, third-party tools (e.g. Corona SDK) don't support it until it's officially out, because part of the NDA you have to agree to before you can download the SDK beta says "you can't talk about it publicly". 

But now that it's officially out, and officially supported by third-party tools, I feel that I have to start working on it. Not yet urgently, but soon. 

If the feature set of the latest iOS version is not a consideration, and my only objective is to "target the platform where the most iOS users are", when should I target iOS 5? Up to when should I support iOS 4? iOS 3?
On average, how fast do iOS users upgrade to the latest version?
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Until recently, I have been happily ignoring iOS5 SDK because its new features aren't really essential to the products I'm working on. Also, third-party tools (e.g. Corona SDK) don't support it until it's officially out, because part of the NDA you have to agree to before you can download the SDK beta says "you can't talk about it publicly". 

But now that it's officially out, and officially supported by third-party tools, I feel that I have to start working on it. Not yet urgently, but soon. 

If the feature set of the latest iOS version is not a consideration, and my only objective is to "target the platform where the most iOS users are", when should I target iOS 5? Up to when should I support iOS 4? iOS 3?