I've asked this question, and heard a few tips in RIU ranging from "anytime" to wait until the plant starts putting out leaves in alternation from each other (they start out positioned exactly opposite from each other, then become alternate later).
I like the "6-8 internode" rule someone gave me, mostly 'cause my plants are just now passing 6 internodes! My plants are about 9-12" right now, and are still putting out opposite leaves, so that rule leaves me waiting...
The weight of yield is influenced not only by plant size, but light intensity. If your light level is marginal (say.. less than 40 lumens per square foot), your buds will be lighter and fluffier (but still good smoke, so no despair..). Getting your light at 50 lumens sq ft will give you harder, denser, heavier buds for any given size. Growing smaller plants more concentrated under your light is likely to produce as much or more yield than bigger plants struggling to get their fair share.
