Depends on a number of factors like ambient humidity, do you sleep with your windows cracked, etc...

I try to apply it once a week, and use it in concert with rain guards (another $25 amazon purchase that made a HUGE difference)

The end result is less than perfect but still quite an improvement. With low humidity and cracked windows, you can wake up without any interior fog at all. With active rain and cracked windows, you still get 1 - 4 hours before fog starts (long enough for anyone who might notice fogged windows and object to a car camper to forget about you. Perfect for attempting a new residential parking spot overnight or spending a few hours in the back of a parking lot without drawing any attention). If you are running a portable fan, even better.

Either way, it's lower profile and less time spent blasting the heater first thing in the morning.