We harvested the sunflowers. The seeds barely have the black around the edges and the backs of the heads aren't yellow yet, but I saw a goldfinch darting in and out stealing some, so it was time.
There is one big one still out there that's still perfectly white (the seeds) so I left it for now. I also left two small sunflowers for the birds to enjoy.
Hey, they let me know it was time, so I figure I owe them a few!