Generally, streps require 8-12 weeks to ripen their seed pods after pollination. Once the pods turn dark they will usually start to crack open, but if this happens before the 8-12 weeks has passed, the seeds are likely to be no good. Because your seed pods are 16-18 weeks old, I'd think they're ripe by now, but just didn't crack for some reason. I haven't had this happen before though so I'm not sure.

Mark

In order not to miss the seeds if they scatter all of a sudden, you might tie a plastic bag over seedpod. If seed fall into it, you will know it is ready to be cut from the branch.

Nancy