Length is probably anywhere from 500-1500 chars.
Is saving that much text a misuse of the keychain?
Storing a few KiB in the keychain is fine. However, there is another serious problem to worry about here. You wrote:
In a new app, a requirement is for the app data to be stored securely by the app and be able to survive an app delete/reinstall.
The keychain does not guarantee that data will survive when the app is deleted. Please read this post for the details.
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Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware
let myEmail = "eskimo" + "1" + "@apple.com"
WWDC runs Mon, 4 Jun through to Fri, 8 Jun. During that time all of DTS will be at the conference, helping folks out face-to-face.