Both of our 1998 MB's have the 722.6 tranny. The dealer INSISTED on the lifetime fill.
At a service clinic, the Zone Rep was out, and I cornered him on it. He mentioned that if it were HIS car, he'd change it every 90-100,000 kilometers. (55-62,000 miles)
I had it changed at 90,000 kilometers on the C230, and the fluid and filter looked brand new. BRAND NEW! I'm used to doing tranny services every 50,000 kilometers, and have never seen fluid come out looking brand new.
I'm going to keep doing it every 90,000 kilometers, just for peace of mind. I'm probably wasting some money, but $275 every three years or so doesn't seem too outrageous.
One thing to ensure is that it is done properly. The dealer followed a very precise process that had the fluid at 80C when they measured the level. I didn't notice any changes to shifting whatsoever after the change.
When the turbodiesel hits 90K, I'll have it done. And I'll betcha it looks like new...
1998 C230 "Black Betty"