How do you know when mussels are done? I bought 1/2 pound of mussels and cooked them in a cast iron skillet with a little white wine and some shrimp stock on boiling heat.

Almost all of them opened after about 15 minutes, some did not. The ones that opened were done perfectly, and those were the ones I served.

Still, in the future - should I just throw away the rest, or would a few more minutes in the skillet be in order?
You should throw them out. Mussels that don't open were quite possibly dead and decomposing for an unknown period of time prior to cooking. You don't want to take a chance here.

Just in case future readers don't know, mussels, clams, and other in-shell shellfish are still alive when you buy them, and they should be still alive when you cook them.