Mike Miller is recommending 5k intervals for the N54. I think it will depend on your driving style and conditions (temperature, etc.). But I think 15k is too much unless you do mostly highway driving and live in in a warm climate where it rarely gets colder than 50º F.
I did an oil analysis on my E92 335i's oil at 4.3k interval. This was the "stock" oil. It was still in good shape and Blackstone said it could go another 2k miles. But I live in SoCal and drive 16+ miles each way on my commute, getting my oil fully warmed up to 240º F.
Im going to do two 5k intervals to clean the engine out, then switch to 7.5k intervals confirmed with oil analysis.
I plan to keep my 335i and expect it to run just as good at 100k as it does now at 47.7k.
2009 335i 6MT
(E92/N54), Black Sapphire / Shadowline (Individual 760) / Black Dakota Leather, ZSP, ZCW, 6FL, PDC, no Nav/iDrive, 19 x 8.5 / 19 x 9.5 VMR VB3 in Hyper Black, Pilot Super Sport 235/35 fr 265/30 rr, M3 control arms + tension links (fr)