I've looked at the Message (email) class in the Office 365 API Resource Reference, as well as the other classes and various Office 365 docs, but there doesn't seem to be an elegant method for obtaining the duration a user spent composing an email. I'm very surprised that the Message class does not have a "ComposedTime" attribute or something along those lines, for emails in the SentItems (Sent) folder.
My current implementation is very brute-force, I am hoping there is a more straightforward implementation, thank you!
The back end doesn't track that, so no, there is no property for it. You can always suggest it on UserVoice.