honestly, it all matters on how you drive. if you tear the shit out of your car regularly, i would change much, much sooner than the 15k. I run a JB3 on Map 5 and drive pretty hard. I notice a change in performance if I let the car go without a change any more than 7,500 miles, furthermore, the engine runs a lot hotter with the JB3 and a higher map setting. Generally my rule of thumb is when the engine oil becomes low after a change, I add some, then when it's low or at the minimum again, I change it. This is usually 6k-7.5k for me.
Now, if you drive a 328 or something other than the 35 and don't drive that hard, I see no harm in waiting until 7,500-10,000k for a change. I had a 328i before and would change every 9-10k.
'07 E92 | Space Grey | Sport | Premium | JB3 | aFe DCI Stage 2