Actually, the ATF is "supposed" to be good for the life of the car, or the better part of it. If the trans needs service, no lights come on or anything, only the dealer will know when you take it in. (What happens if you don't take it to dealer? I don't know.)
There have abeen quite a few discussions on this topic, try doing a search on it.
I believe John, blackmercedes, has his ATF renewed at every 60k miles, but I'm sure he pays for it.
2006 E350 w/ 155k miles (Daily Driver)
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)