When do you replace the timing chain on a BMW 520D model 2008?

Normaly you don't change the timing chain on BMW 520d model 2008 it supposed to last for life but the 520d model 2008 it's prone to a timing chain failure ( Design fault from Bmw ) which bmw want recognise that ! But there is a lot of people was affected ! 4% of those cars are affected . If you heard any strange noise like rattle or ticking noise coming from the back of the engine , go and check it at a Bmw Dealer , make sure to have a warranty because if it fail it cost to repaire it almost 4000£ and if it snaped you will need probably a new engine or second hand one ! Most of N47 engine from 2007 to 2011 are affected ! more info google N47 timing chain failure.