Ripen plantains in a loosely closed paper bag at room temperature. It takes six to eight days for a green plantain to fully ripen. Fully ripe black plantains should be firm like bananas, but not hard. Do not use plantains if they are squishy, moldy, hard, or cracked. Do not refrigerate them unless they are at just the stage of ripeness you want to use, because the chill will stop them from ripening further.