It regenerates both ways, distance traveled and pressure difference, whichever comes first... It does have a sensor that reads the pressure difference between the pressure before the DPF and the pressure after the DPF. That is how the true soot level/value is calculated. The gauge on the instrument cluster might not be quite accurate in all the cases since it calculates the distance traveled also. The gauge might show 100% full due to traveled distance, but the true value calculated by the pressure difference can be at 6-7g/L (6-7 grams per litter) 75-80% full. 100% full based on the pressure difference = 8g/L. It would be good to monitor both, distance traveled and the pressure difference value at each regeneration...
Also, remember to keep your DEF tank full in order to prevent the triggering of the start countdown, as me and you discussed it a while back on your Youtube channel.