BigBear, 3 seconds minimum. Back when I took driver's ed, it was 2 seconds; I've been told that they've realized since then that 2 isn't a long enough period, and increased the teaching to 3 seconds.
Although in most metropolitan area traffic, that doesn't work. If you are 3 seconds behind someone, someone from another lane will cut in front of you into the space you created--forcing you to slow down to get 3 seconds behind that car. Which causes another car to cut in front of you, which causes you to need to slow down to get 3 seconds behind that car, which....
It gets to the point where you need to drive in reverse simply to maintain a 3 second separation.