What is the cost for Timing Chain Assembly replacement and how often/mil should?

Hello, a timing chain is installed for the useful life of the engine, and has no maintenance requirements or expectations. That said, there are causes of failure, so it's hard to know what is going on without a diagnostic inspection. For example, there could be a timing chain tensioner, which maintains proper tension of the chain, that is failing. Totally a guess and can't be sure. An experienced BMW technician near you will be able to tell you a lot of what is going on during an inspection. Experience and training means so much!