Has there been an end date established for the Maps app on iOS less than iOS 6?
As long as you do not upgrade to iOS 6, you will get to keep the app forever. As for the API not working any more, since Apple used the official API any changes Google makes that would break the old Maps app will also break millions of other apps on many platforms. There is a possibility that Google could revoke Apple's API key, but I seriously doubt that will ever happen.