Not sure what the interval is, but the proceedure is pretty simple.
Remove the return line from the resevoir cap, attach a piece of clear tubing to the line and put the other end in an empty milk jug, start the engine. The pump will begin to pump the fluid out and it will collect in the milk jug. Keep the resevoir topped up with new MB hydraulic fluid and left it run until clean, clear new fluid is passing through the clear tube. Shut the engine off, replace the suspension filter and reattach the return line. Should take about 2 quarts of fluid to purge the system.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.