personally, i think never. even if they get a little light, they are making more juice for the plant. but if its a crowded mess, and air flow sucks, and theres no light, then ya run a risk of rot and mold down there, and then asap is the right time. (keep in mind theres shock involved, dont do it soon before/after a transplant, a water flushing, etc). i also tie down the main cola's at a certain height so all those lower branches become big and juicy. depends on what ya want tho, 1 big nugg, or a lot of small ones... depends on the size of room ya got, the air flow, the lighting, the humidity, and how ya feel overall. fuck you want it to look like a christmas tree you chop it that way. whatever