Look at your trichomes. Use a 60x jewellers magnifying glass to inspect the colour of the resin heads. You'll see Clear, Cloudy/Milky, and Amber colours.
Best time to harvest in my opinion is when you have the following ratios,
70% Cloudy
20% Amber
10% Clear
I would say you need to begin flushing your soil of nutrients. Just plain water with the proper pH. I like to add a clenser product like GH FloraKleen. Every nutrient line has their version. Flush till your plants start to yellow and show lack of nutrients.