Personally I'd wait until the trichomes have turned..the 7-8 weeks is only a guide..and if you are growing outside, this varies with your geographical latitude...the rate at which night turns to day and vice versa affects the ripening.. For example... I fooled my plants into an earlier and shorter ripening time by moving them in and outside a month before the normal flowering season but with longer periods of darkness of about 14/15 hours..once normal flowering of the other plants had started I stopped doing this and left them outside to finish off naturally.. But by doing this I've shortened the flowering period from 9-10 weeks down to 8 weeks, and have them about 6 weeks early...although the harvest weight is reduced.. So, with tinkering, these guide figures will vary... The further north you are the longer the flowering season...