Answer:
The weed killer atrazine can take 14-21 days to kill weeds. It works by inhibiting photosynthesis. It has to be absorbed by the plant (hopefully weeds) to stop photosynthesis; this is why it can take so long to work.
Many people expect results within 3 days so they often cause damage to their grass via over-application. Two things that may effect the time it takes for noticeable results are: 1)the amount of atrazine the product contains and 2)if the product is time released, such as many of the weed and feeds (a fertilizer combined with weed killer).