If you wanta good starting point for an educated guess. I have always used the rule...

When 80% of the hairs are Red its safe to cut her down... Or cut the top off and let the smaller buds finish for aother week/ Or if all the hairs are Red its still safe cut but it has been maybe a little too long.

I've always used that kinda thinking and I have never grown weed that hasn't gotten me messed up!!!

kd