My general rule of thumb is that I start using it when the car starts to struggle on start up. This may or may not ever happen for you based on where you live and where you store your car (garage or outside, etc). For me this is usually around the freezing mark (32 F).

I have a timer that goes off around 3 a.m. and heats my cars for about 3 hours til we leave around 6 a.m. Toasty warm with instant heat!

But my two girls are at college now and one of the cars is a bear to start in the winter. I believe that we will be idling it most of nov, dec and jan.

Don

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DAILY DRIVERS:
'84 300DT 298k (Aubrey's)
'99.5 Jetta TDI IV 251k (Julie's)
'97 Jetta TDI 127k (Amber's)
'97 Jetta TDI 186k (Matt's)
'96 Passat TDI 237k (Don's
'84 300D 211k Mint (Arne- Undergoing Greasecar Conversion)

SOLD:
'82 240D 229k (Matt's - Converted-300DT w/ 4 speed