This is my first winter with my 2013 Passat and I too have been trying to figure out what is 'normal' to expect for heat in temps below 0c.
Mine takes an extremely long time as well to begin to warm up and I generally understand the difference in the way the diesel generates heat compared to a gasser.
I will be checking for the relays in my car tonight as I also do not seem to be getting any benefit of an aux heater if there is one installed. Given I have a Canadian model, I would expect that there is one, but I'm not sure it is actually working.
At temperatures below 0c my car seldom reaches operating temps of 75c, unless it is doing a regen in which it will reach 85-88c, and then quickly drops back down when finished. I am suspecting a stuck thermostat, but I also understand that 70c is within the 'range' of the factory unit, so not completely sure.
Needless to say, keeping the chill off and the front window clear is a bit of a challenge.
Is anyone aware of a monitored PID or any other way of indicating when the aux heater is on? I use the Torque App and if there is a way, it would be good to have some kind of indicator.
Any suggestions on what might be going on, or what to look for would be appreciated.