Good old nirvana!! Most of my seeds come from there, love the service.
JustP - are you growing in soil? what do you feed with? and do you feed every watering? Do you use any soil conditioners like molasses or zymes? From those pics, It is hard to see, but doesn't look like many amber hairs (this may be a white widow thing - anyone chip in here?) which is one indicator, but not the best. Nirvana recon the plants at 8-10 weeks flower, and from my experience the later is usually closer, especially if you are countijng from the day you turned to 12/12 (MJ needs time to adjust before REALLY being in flower).
It doesn't look like you have a lot of nute burn going on, which is a good thing and they shouldn't need too much flush. I would say at the earliest week 7...but get back to me on the other factors, and I'll give you a better idea.
I din't write the book on growing MJ, so may be wrong, but I have put a lot of over my last few grows on flushing and finishing times