Start trying to force the herm right away- early in flower- so not only does the plant have time to make nanners, but the seeds produced have at least a month to develop. Seeds need a good 5 weeks on the plant; they WILL continue to ripen slightly while the plant is hanging to dry, but a month on a live plant is about a minimum. If you want to be sure, let the plant ripen alive until the seeds are FAT and BRown, and the calyxes containing seeds are tan colored and papery, regardless of trich color.