6 weels is a good starting point. If they are falling out of the calyxes early, you can harvest. (exaggeration... if they are fully developed before that point, you don't have to wait, obviously). better to be safe than sorry and have large % of inviable seed.
if you allow it to be pollinated over and over across a longer length of time, you have seeds developing at much different time frames, so try to douse them over a shorter period of time and remove the pollen donating plant / clean up as best you can, though no need to be anal about it. wipe down walls and floor / platform etc. get that excess pollen out of there so you can have a single harvest date and get 95% of the seed with the same development time.