I presented this exact question to Todd Hill at VolksWerk a
short while back and this is the reply I received and saved
for future reference:

"How long will it last?  As you stated, thats fairly open ended.
I have seen anywhere from 60K to 260K on transaxles before
they need repair.  As a very general rule of thumb I would think
that 150K+ is an average life expectancy on a Vanagon trans.
Perhaps a bit less on the Syncro due to the added complexity
of the Syncro stuff and weight of the vehicle.  "