From the time of pollination, allow 5 weeks before the seeds are ready to harvest- that is for a typical commercial 7-9 week strain.
When the seeds turn fat and brown and striped, and the calyxes look tan and papery, you can cut your plants and hang them to dry. Put a white sheet on the floor to catch seeds that fall out in case any do.
If your plant is still not ripe by the trichome ripeness standards, it is fine to let it ripen more. That won't hurt the seeds. Just give them at least 4 and preferably 5 weeks (if you care about them) on the plant before harvest.