It is recommended that you change your timing chain every 100,000km. Over time the chain can stretch and can eventually break, so what you want to do is change it before it breaks. Depending on how you drive the car, sometimes the chain will outlast the car without a problem but as a good safety measure it's good to check it every now and then. I just had mine replaced, it was 3cm longer then it should have been, and I could hear it rattling on a cold start and when the engine was running, on the 560SEL they call this the rattle of death, because if the chain breaks the engine will be killed. Start your car and and listen near the timing case to see if there is rattling coming from it, if so, then start thinking about changing the chain soon. Also make sure you get a good quality chain, some aftermarket ones can break very quick. You can also take off the valve cover and see if the chain is loose, this is how I realized that mine had to be changed.