It would make sense that the seeds are mature when the plants are mature.
I have grown many seeded and hermie crops and you'll know when they are ripe. Some of the top ones will actually fall right out of the pod (just like in nature). Others will be bursting from the pods but not quite falling.
I imagine seeds from different strains would look different but my mature seeds were always a nice dark brown color with spots and mottling on them.
Here's a shot with a nearly ripe seed poking out of the pod.
I would also recommend just taking the plant out to its normal harvest. You can take seeds too early, but they do not have an "expiration period" or a harvest window like bud, so it would be hard to take them too late. Three weeks would be an absolute minimum and even at 3 weeks, I have found many immature seeds. Just let the plant do its thing and harvest the seeds when you harvest the bud.