From reading the below 2 forum posts I have deduced:
The recommended brake fluid change interval is every 2 years
This could be extended by changing out some of the brake fluid by spooning out of master cylinder or better bleeding out at all wheels and topping up the master cylinder every year.
This would probably allow a 5 year complete flush interval.

This posts shows how you can test for moisture to see if fluid needs changing
http://sprinter-source.com/forum/showthread.php?t=731

And this posts discusses at length many options including the one above over 6 pages:
http://sprinter-source.com/forum/showthread.php?t=40690&page=1