Wait a minute... My '97 S500 manual says it has a lifetime fill! Thery're right. The fluid will last exactly as long as the transmission! In my car's world, that means about 120K per transmission/oil change.
Actually, since this policy is so rediculus, by the time you buy one of these cars with 80 or 90 thousand miles on the meter, the fluid is well on it's way to oblivion. I change mine every 25K in hopes I can nurse it along until about 180K. (car has 101K now)