DPF regen is triggered when a certain pressure is reached across the DPF as monitored by the differential pressure sensor (basically, the higher the soot and ash content, the higher the pressure). From personal experience, once the threshold pressure is reached, it can be triggered at any engine temperature but only once a certain speed is reached (not sure what speed but the threshold is definitely between 15-30mph).
You'll be able to tell that the car is undergoing a regen if the idle RPMs are stuck at 1000 and, if the car is below operating temp, you'll notice the engine temperature increasing to operating very quickly.
