They do a DPF regen every 200 miles or so, lasting about 8 mins. Try to avoid interrupting the process by stopping the engine, which causes diesel to be dumped into the engine oil sump. There's an X mark on the dipstick above the normal max mark - if it reaches that level you need an oil change and the car will probably throw some fault lights etc. The tell-tale that it's doing a regen is the rediculously low instant mpg, and the engine light on the i-stop screen goes out (but that light also goes out if the engine is still cold etc). There's more tell-tales, but those are the easy ones. I try to keep the revs above 2k when mine is doing a regen, to help it along.