Depending on the strain it will take 45-70+ days of budding. I like to cut my plants down when 2/3rds of the hairs turn red. If you look at the crystals (trichromes) under a microscope they should look like tiny little mushrooms. If the trichromes turn grey or brown you have waited too long. Half cloudy white and half purple is perfect. When the hairs are 2/3rds red is usually just about perfect for those without a microscope. The swell is another thing I look for. I want the bud (catalist SP?) to swell like it could have a seed in it. If not then its probably not ready and will be air bud if you cut now.
Getting close to harvest time!
This buds for me!