When you "feed" a plant, there's an X amount of days worth of food for it to uptake.
What I do, when I anticipate my harvest on a certain day, I plan my feedings to correlate with that harvest day, so by the time the plant "runs out of food" after it's "last feeding", viola, it's harvest time.
All the nutes in the plants are metabolized, there are no wasted nutes, and no chemmy flavor in the buds.
It's all about timing and reading your plants needs.