I planted a dozen or so and the heads are heavy with seeds, but they still look white vs. the black color of normal sunflower seeds.

When do you cut them and do you hang them afterword?

Spot

Have they drooped over and dried out? Yes I hang them in a dry place. The big heads are white and grayish in color

Yes the are dropping over big time, and starting to dry out, the blossoms on them have been dry for a while

Time to cut and hang in a dry place.

What are you going to do with them? Make your own salted sunflower seeds? I'm curious.

I saved mine for the birds in the winter.

Seems a little early. We combined about the same time you combine corn. The leaves will have dropped and the stalk turns brown. I suppose if you're hanging them dry, you can cut them earlier.

If you're growing them to feed the birds, just leave them, the chickadees here find them when they're ready, and stash them somewhere for the up-coming winter. Saves a guy a lot of work!