Personally if it was me, I'd wait at least a year, if the weed killer and fertilizer is not organic. I wouldn't want to grow anything at all on sprayed land, but that's me.

If it's an organic herbicide and fertilizer, depending on what it is, you might be able to plop down mushroom manure over it after it wears off (the weed killer) and then go from there. Maybe do a 'test' germination of some seeds you don't care about just to be safe.

You might still be OK, assuming you weren't planning a certified organic garden. I say MIGHT, because to answer your question with certainty, we would need to know what product he used, what the active herbicide in it was.

Weed and Feed products are generally post emergent (they don't prevent germination of weed seeds, they kill actively growing weeds), are often but not always 'non' or 'low' residual - that's why they are applied directly to weeds already present (contact).

"Although nonresidual, some herbicides may remain active in the soil for a few weeks ("carryover"). This time period will vary with the herbicide. The information should be found on the label."

Thanks for your responses! It was Scott's Turf Builder & +2 Weed Killer. I think I'm going to just try it- I called Scott's and they said the weed killer doesn't have a pre-emergent in it, so the seeds shouldn't be harmed. We'll see! The seeds I'm throwing in there are just for fall decoration items, so while I would have preferred not to have any additional chemicals in there, all the corn stalks and pumpkins are just going to end up decorating the front lawn come fall so it shouldn't really matter.