BMW's plan was for it to never be changed. Looking at some of the forums, I think advice given from this BMW site seems about spot-on:
Well-l-l, I hate to say "never" but the point being, one often needs/wants to crack open & rebuild an engine for other reasons (ring/valve wear, main seals, etc.) by a quarter-million miles or so--at which time, check the chain while you're in there. Otherwise, yeah, effectively lifetime.
Properly lubed, the t-chain and tensioners/guides will last as long as the crank bearings. When the engine is eventually torn down for a rebuild, a chain kit (and crank and cam gears) is part of the job.
Until then, be happy we don't have timing belts!
On the forum, the sentiment seems to be there aren't many instances of a broke/slipped chain. They just don't normally fail. I do agree with the @JoshCaswell thought & link above ... preventive maintenance will never serve anybody wrong.