The factory now agrees.  100,000 miles.  I'd recommend 2-3 times more often
than that.

R / John

The trick fluid manufacturers recommend a 100,000 mile life. The car
manufacturer cynically believes that although the fluid performance declines
at around that point, that the transmission will continue to function,
albeit with an accelerated rate of wear, for long after that, so they will
have few complaints if it fails at around 150,000 miles or so.
Who knows, perhaps the fluid is not the limiting factor anyhow and
irrespective of fluid changes it would fail at the same point, at which it
would need an overhaul and a consequent change of fluid anyhow.

Personally I would change the fluid at around 80,000 to 100,000 miles
whether it needed it or not, assuming I wished to keep it for much longer.

Huw