Like said before, pistils are an obvious and easy sign to tell if your getting close to harvest.
Your pistils are basically all white still. When the plant is done, it will look swollen, the pistils will turn orange/red/brown, and will look like they are receding back into the calyxes.
The best way to tell if a plant is ready for you is to just get a scope and check the trichs, and harvest on your preference.
To answer on when to harvest, I would say 5-6 weeks. It really depends on when it start showing it's first pre-flowers.