In my experience the seedpods will not break open (dehisce) on their own while on the plant.
I simply let the pods grow until they turn brown and dry on the plant, collect, and open by hand.
If you were to harvest when the seeds pods are nearly done, turning darker but still a bit green, and allow to dry inside, I think they may still be viable, but seem to be a bit immature and a little less healthy than the fully formed seeds when the pods are completely brown and dried.
Ideally this stage or a little later. I tend to let them dry out even further on the plant. (from the
Desmanthus Workspace Thread)