In Vancouver you'd be fine with 2 hours, max.
300-400 watts isn't a lot for a block heater. They go up to 1500w.
But you do need to manage it with a timer. Or if you're not on a regular schedule, you can get a remote switch so you can turn it on from inside.
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To keep from driving away with the thing plugged in , loop the extension cord around your driver's mirror when you plug it in.
I've
almost pulled away with it plugged in once (didn't have it looped on the mirror
), started moving then remembered at the last minute: "Oh @*#&!"