I don't know the specifics for BMWs but pretty much all latest-generation diesels with DPFs regenerate both when the pressure sensors indicate the DPF has reached a threshold level of soot, AND on a mileage basis (the latter is a failsafe, really).

The DPF will be very close to the turbo - it has to be in order to get the temperature high enough for 'passive' regeneration, and for post-injected diesel to burn in the matrix to raise the DPF temperature and burn off soot during an active regeneration.