If the Block Heater is plugged in.. It's continuously Working/Heating. No thermostat.
All the more reason to have it on a 15 amp (or better) timer, with at least a 12 gauge (short as possible) extention cord..... Timer set for the amount of time needed (depending on your local temps) to provide a [bit] of block heat to [aid] in a cold start.
A longer ON time is not Better. [Enough] time is the key to it. What "Works", for You.. and Your.. PSD... based on outside temps.
Joe
Ever notice you need to add a little coolant in the Winter, but not in the Summer ??
Thats cuz the Block Heater "Perks" a bit of the H20 out of the coolant mix ratio... but not the Glycol.