Hilchos Shemittah v’Yoveil 5:15

One may not harvest shemittah produce before it ripens as per Leviticus 25:12: “You shall eat its produce,” meaning that it may not be eaten until it’s considered produce. One may snack on a small amount of unripened produce in the field, the same as one does in other years. One may not bring the produce into his house until it reaches the tithing stage.

Hilchos Shemittah v’Yoveil 5:16

It becomes permitted to eat shemittah tree fruit in the field as follows: Unripened figs may be eaten with bread in the field when their exteriors get shiny. Unripened grapes may be eaten in the field when their liquid drips. Olives may be broken open and eaten in the field when a seah of olives (about 2.5 gallons) would produce a reviis of oil (about 3.5 ounces). If a seah of olives would produce half a log of oil (about seven ounces), one may crush the olives and anoint with them in the field. When they reach the stage where they would produce a third of their full output, they may be brought into the house because they have reached the tithing stage.