Default time is 15 minutes then it auto shuts off. You can switch it to stay on infinitely. I haven't had any problems with ice/snow here in CT, it seems to melt it fine. If you let it shut off after 15 minute though, it will start to fog up again.
How short a time do you expect it to be? Lets not forget if TOO much heat is applied to frozen glass, it will shatter. Hence - The electric heating elements are designed to be gentle.
If my rear glass was COMPLETELY covered with 1/4 inch of ice, I would expect it to take at least 40 minutes before I could remove the ice. Since I have driven a truck with NO rear window, I am OK with using side-mirrors when the rear window is iced.
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5th Gen Subaru Impreza Forum
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