Ruby red grapefruits do not have a set ripeness stage; they gradually improve in taste and increase in size as they mature on the tree. You can check the readiness of this fruit as early as September throughout the southern United States, and it can be harvested through May.
Horticulturist Julian W. Sawes recommends removing grapefruits from the tree in its first and second year. This helps the tree focus all its energy toward growth, which could increase its fruit production in subsequent years.
A green peel in the early months is not an indication of whether the inner fruit is good to eat; taste one at a time over this period to see if the grapefruits are mature. A grapefruit that feels heavy for its size has a lot of juice, and one that is overly ripe has wrinkled skin.
Ruby red grapefruits will remain edible for two to three weeks if they are refrigerated. Unlike some fruits, grapefruits will not continue to ripen after they are harvested from the tree.
Grapefruits are no longer good for harvest once they begin to fall from the tree or when the inner seeds begin to sprout. A late harvest can result in less fruit production the following year.