Depends on the State and city. Apparentl Federal Guidelines say between 3-6 seconds, but nobody really has to follow those guidelines.

Here's an article from Chicago you might find interesting/helpful:


Yellow lights in city shorter than in suburbs - chicagotribune.com

Here is another interesting link; however, the times they recommend have no legal status, but do seem to match Federal guidelines, but additional speed information:

Yellow Light Time Standards | Stop Short Yellow Lights