I have them on my Cessna 206 and they get used regularly all winter long, whenever it is below freezing, which means October through April.  They are about 6 years old and I think that two of the bands have stopped heating, but I have not gotten around to ordering replacements.  When I got them, they only had the lower wattage, but that is plenty if you have an insulated engine cover and leave them on over night, even at -20F.