I just saw this in Ed Rosenthal's guide:
(... upon planting a germinated seed in it's own pot...)
After a few weeks of growth, a branch is
clipped from each one and marked. The clipped
branches are kept in a jar with water and a few
drops of houseplant fertilizer, then placed under
a regimen of 10 hours of light and 14 hours of
uninterrupted darkness. Within 7 to 10 days the
plants will grow flowers, indicating whether
they and their clone mothers are male or
female.
This is the fastest claim I've heard of. How it pans out in practice...
E4A2S