Every 96,000 km.
This procedure is not included in the manufacturer's maintenance schedule, but is highly recommended as a precaution against timing belt failure during operation. If the timing belt fails while the engine is running, serious engine damage can occur.
See Part Repair work carried out without removing the engine from the vehicle, Chapter Removal, inspection and installation of the timing belt and its tensioner sprockets.
[The website served as the basis for the text: BMWman]