For that kinda car about <5 years. Look at the 750Li. Many at the <2012 mark that are 20k. 2013-2015 in the 30k. Less if they haven't fixed up the mechanical trouble. The Ultra Luxobarges tend to suffer terrible(Great?) depreciation. The LS isn't immune either but does a bit better.
I'll even give the RLX honorable mention for terrible(Great?) depreciation.