For "conventional" hydraulic trannies like 722.3 or 722.4 (four speeds) then it's fluid/filter every 30K-miles or 50K km's.
For the 722.6 five speed fully electronic tranny, the factory says "fill for life" but not many people who keep their cars follows that. Consensus is that 50,000 miles/80,000 km's seems like a "good" interval for the 722.6.
In North America, the 722.6 became model-wide for 1997+. Your car probably has that tranny, but I wouldn't say for sure concerning other markets.
BTW, if you have the five speed, you MUST use the MB synthetic fluid. Do a search for "722.6 transmission" and you'll see LOTS of threads...
1998 C230 "Black Betty"