Day 20 from 12/12 flip
Day 21
day 60
All preference.. I like how it fills out mid/lower buds much better and allows more even ripening throughout the canopy. More light to the buds usually means more yield and larger denser buds. I see it as once the plants begin to develop buds it's time to grow bud not big fan leaves.
EDIT: Keep in mind different grow styles will dictate how heavily you can defoliate, I notice that after a heavy defoliation my plants eat A LOT more for up to a week, as high as 2.6 EC with once daily watering feeding Maxibloom. It's worthwhile to consider when you pluck off fan leaves you are throwing away a lot of energy the plant has stored so if your in soil for example it would probably be smarter to be more careful in how heavily you defoliate.
I personally remove all the large/medium fan leaves from the plants branch by branch on day 20-21 of flower. I have never had a plant hermaphrodite from this practice and or die or anything crazy. The only thing I've noticed is increased yield and also my dehumidifiers run a lot less without all the extra foliage.