In my first 7 months of ownership I never did notice a re-gen. The exhaust tips were always spotless.
The reason was it wasn't doing a re-gen because the back-pressure sensor had failed, which over time started to cause a significant drop off in power.
I had the sensor replaced and a forced re-gen, which took a couple of attempts by
PMD at Chelmsford.
The exhaust tips now get reassuringly sooty, but I can't say I'm ever aware of any signs of a re-gen taking place except for the occasional 'eggy' smell.