I too have discovered this behavior while attempting to dictate in Apple Mail.
I would just like to add that this behavior is juxtaposed to what iOS dictation does. When an iOS dictation times out, it captures and transcribes the previous 40 seconds worth of dictation.
In and interesting twist, I began dictating this paragraph (using Safari on a MacBook Pro with iOS 10.8.3) and I watched the time. While my iOS device (4s & 6.1.2) can record for 40 seconds and, in any app, faithfully transcribe my verbal input, indeed this recording timed out at 30 seconds. What's interesting is that my transcription did appear after it timed out!
So, we now know that this behavior is specific to certain apps (like Apple Mail) so, I will go now to Apple Mail preferences to see if there is a setting I can change. This may be a security related setting.
I'm back and I have this to report:
There are no dictation preferences in Apple Mail. So, I went to system preferences and reviewed the general dictation preferences. nothing there that was relevant.
However, when I went back to Apple Mail, after dictating here in this Apple discussions forum using Safari, Apple Mail suddenly began remembering and transcribing my text!
So, clearly there is a bug and I will attempt to report same to Apple by linking to this discussion but it is a self repairing bug – the best kind! ;-)