In cold weather on a cold engine, I don't think you run too much risk of burning it out. Its roughly equivalent to a coffee pot element, and those can last many years boiling water - you won't be boiling any water.... Its a lower wattage than a coffee pot.

Just a guess. Have never used the block heater(s).

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles