You already have a ready seed bank in the soil now. Don't worry about gathering too many seed heads. Since each "flower" is actually a hundred or more individual flowers in the center "button", you will find that they don't all bloom at the same time, so they don't all mature seed at the same time, either. (That's why each "flower" (with the yellow petals) lasts such a long time.) Consequently, you'll find there is an equally long time that you can gather seeds. In other words, it's not rocket science, and you'll be good. I usually wait until the stem begins to dry for maximum seeds, but as I said, you'll be good if you choose earlier.
Be aware that, any plant of the Asteraceae family (including Rudbeckia) tend to have a high percentage of seed that is actually non-viable chaff, and likely different from what you are seeing drop out of the seeds heads now. So gather 10x more than you were planning.
