I'm not familiar with the "Extended Roundup" product. Check the label to see if it is Roundup with a pre-emergent weed killer or 2-4D. If it is a pre-emergent, you can't expect seeds to come up, but you can bed plants.

It takes a while for 2-4D and its derivative products to break down, and if the label says it has 2-4D in it, then you should follow label directions. For a few days, watch the weeds and grasses that are coming up see what they will do. As a general rule, 2-4D is a broadleaf killer, and will not affect most grasses. If your weeds turn brown and melt down, you will have to wait.

To get more information, you might enter the the label's active ingredients one at a time in a search engine such as Google.

Extended Control contains dithiopyr, a preemergent, along with the glyphosate.

Thank you for your suggestions. I will google the active ingredients and see what I find. I do have some chemistry background. I could probably also plant a "sacrificial" plant and see how it fares. I was thinking that if I called the 800 number on the label, they would just tell me to wait the full 3 months in order to not contradict their claims. But I'll try that as well anyway.
Again thanks.