Your best way to tell when to flush your plant is based off the trichomes. Usually once the trichs are 30-40% milky, i will flush.
General rule of thumb is having about 2-3 weeks from flush to harvest. You want to flush with at least THREE TIMES the size of the pot of water. Check runoff and PH before and after.
If your plants yellow out too quickly, resume light feeding till finish. Happy growing :)