When I bought my C230 the dealer insisted that the tranny fill was for life.
Shortly after, I met with the Zone Service Rep and he explained that if I were keeping the car (as opposed to getting a new one within just a few years) then I should change the fluid and filter every 90,000-100,000 kilometers. I had it done at a touch over 90K. (That's 55-62K-miles for you Metric Impaired)
I have it done at the dealer, as they don't charge any more labour for the job than anyone else and they have the scan tool for setting the fluid level while the fluid temp is 80C. It also means I don't have to buy the dipstick.
Since the job is done only at such a high mileage interval, I don't mind paying the additional charge for the dealer to do it, as opposed to DIY. On a per-mile basis, it's cheap...
1998 C230 "Black Betty"