I prefer the third/fourth week of true flowering (day 21-30). Most of the central calyxes in the buds are mature enough to be pollinated by then, and it usually results in nice well-done seeds by the end of flowering. The top parts of the buds stay unaffected, while the center and lower halves develop nice seeds. That way I can harvest seeds and buds at approximately the same time.
Any sooner and usually 50% of the pollinated calyxes don't set proper seed, much later (week 7 and beyond) causes a huge developmental difference in the seeds; half of them will be done, sometimes even germinating after some rain, and half of them will not have developed enough to be viable.
I do it by gut feeling nowadays. Just before the plants are halfway flowering.