If you want my opinion, the answer is never.
Weed and feed products are not the best way to go. If you need a week killer, get a stand alone product. If you need fertilizer, get a stand alone product.
Actually, the problem with weed and feed is that it really is not timed correctly.
The best way to kill weeds is when they are actively growing. To do that, you want to fertilize the lawn days or a week before you apply weed killer. Weed & Feed tries to kill the weed before it has been able to absorb much of the fertilizer.
If you ask me, weed & feed is a great marketing ploy for the big lawn care companies. But it is not the best way to handle weeds in your lawn.