The real limits for the alert text are not documented anywhere. The only thing the documentation says is:
In iOS 8 and later, the maximum size allowed for a notification payload is 2 kilobytes; Apple Push Notification Service refuses any notification that exceeds this limit. (Prior to iOS 8 and in OS X, the maximum payload size is 256 bytes.)
This is what I could find doing some experiments.
Just as a reminder here is a very good note from the official documentation:
If necessary, iOS truncates your message so that it fits well in each notification delivery style; for best results, you shouldn’t truncate your message.
~ Answered on 2011-06-11 12:50:58