When do I need to use my block heater?

Just curious - when do I need to plug in the block heater? Previous vehicles I have had with block heaters installed, I generally only used them in winter in very cold places (i.e. minus 20 C or so). Around Vancouver, I never used it at all. My vehicle is a 7.3 L diesel with (I think) 15W40 oil - I heard that diesels need the heater more than gasoline engines do, and I certainly have noticed a bit of roughness on cold starts lately, as we have been having a cold snap (by Vancouver standards - not really cold). Do I just use it based on how the engine sounds, or is there a rule of thumb for this?

It’s used more to keep the oil viscous, it isn’t really to warm the fuel or anything. Friction inside even a warm engine will heat things up real quick. IIRC under 10F oil flows slower, and viscosity starts to go down hill.

I tend to start plugging my car in the day after the first day it fails to start.