Quince fruit trees (Cydonia oblonga) are similar to apples and pears in cultivation, fruit shape, size and flavour. They also ripen at similar times, though their ripeness indicators are different.
Quinces generally ripen in early to middle autumn, especially after a warm summer. Hardier varieties don't grow fruit until late autumn, but run the risk of fruit damage in the event of early freezes.
The fruit skin stays green while still growing in size on the tree. As the fruit ripens, the skin gradually turns golden yellow. When the skin is fully yellow, the flesh will be softer and the fruit sweeter and juicier.
Pick quinces off the tree before they're fully ripe, when they're just beginning to yellow, to avoid competing with birds, mammals and insects for the ripe fruit. Early picking also reduces the chances of losing the fruit to early frosts or heavy autumn rains that cause the fruit to crack. Allow the fruit to ripen off the tree.