Driving down the Santa Monica Freeway I saw that this sign, put up last Friday, was still standing and in fine shape over the eastbound lanes. According to Caltrans, this spot is passed by 277,000 cars every day (Route 10 - Crenshaw) Cut that in two, since it's only visible in one direction, and subtract those travelling at night and you still get a hell of a lot of people. Times eight.
How long signs stay up generally depends on their size, where they're placed and what they say. Smaller signs with relatively apolitical messages like "Support Your Local Credit Union" tend to last longer than their larger more in-your-face counterparts.
Signs are made with scrap cardboard painted white and tracing the text using transparencies and an overhead projector. They cost pennies and take minutes to make.