http://www.mail-archive.com/[email protected]/msg10152.html
I compared the stock of bitcoins to total worldwide wealth, rather than GWP, which is the amount produced in one year.
I think a few more assumptions can get it even higher. Even at our current low rate of wealth creation there will probably be 50% more wealth in 10 years or so. Plus if bitcoin is a better and freer money then wealth can be produced even more quickly. So maybe double that 10 million. Also if people are confident in the money and in future growth they may be even more willing to save in bitcoin as opposed to consuming quickly (on the margin at least).
Ultimately I think this high level will not be fully reach because there will be other good ways of saving value that will be found or created.