I voted never, but in reality, I do eat fast food, but far less than once a month. Only in dire circumstances will I eat FF. Normally on long car trips, or when I have no access to ANYTHING else.
Also, this is subjective. If by "fast food", you mean the major chains, like McDonalds, Burger King, Taco Bell, Carl's Junior, etc, the above statement applies.
I would note that there are many restaurants that have less than optimal food quality, slow service, but still (in my book) qualify as fast food.
-fish
edit: Just realized this is an old thread. Still applies, though.