The seeds are still good after frost if you care to harvest them.
If you allow the plant to mature it will bolt out a seed head. If you wait until the seed head turns brown the seeds will be mature. They don't usually all mature @ once so you can make several harvests per season.
Once the heads are brown the seeds will begin to drop, so you don't want to wait too long to harvest the brown ones.
Set them aside somewhere out of the way and let them dry out well. Once dry just rub a wad of them at a time between your palms and the seeds (along with a litle plant matter) will fall out. They are then easily cleaned using a sheet pan and a playing card or something similar.
Place your goods on one end of the sheet pan. Elevate that end of the pan and while holding it elevated scoop and drop your seed pile repeatedly with the playing card. The seeds will roll to the lower end of the pand while the other stuff stays at the top end.
And no, I'm not going to admit to how I learned this seed removal technique.