I too would love some authoritative info on dosage! Most sources I've found don't say how MUCH to take!
I can, however, advise on when to take it, which is daily. Vitex works in a slow, cumulative kind of way and may have no effect for 3 months. I'm not sure it matters when in your cycle you start it, but I'd think that during or immediately after a period would be best, to give it a chance to "take hold" when all the hormones are low.
Let's hope a vitex expert comes along!