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Except the RISC OS UI really hasn't changed much at all between RISC OS 2.0 and the latest RISC OS 5.19 in development, so that's 24 years.
The differences have been in skinning, basically, and not only that, but the default skin that 3.5 and later have used, is available as an add-on for 3.1x.
Today's Linux desktop environments, along with Windows, give bigger changes in UI available in the SETTINGS DIALOGS than RISC OS has had in 24 years.
Looking at the screenshot of Arthur ( http://en.wikipedia.org/wiki/File:AcornArthur110desktopsmall.png ), the overall model of RISC OS UI even seems already more or less set at this point? (but of course dropping the obnoxious colour scheme)
So that would be 25 years. Either way, basically quarter of a century.