Answer: Lemons take months to ripen and they will hang on the tree for months after they are fully ripe. The only way to tell if they're ripe is by tasting one. If one is ripe, the rest of the crop of the same size and age should be ripe, too. Sometimes they drop from the tree when they're ripe; sometimes they hang on the tree for months after they ripen. A taste test is the only sure way of determining the ripeness.
Since your tree is in a container you should be able to move it to a protected area during the winter months. If you have room, you can even grow it inside. Lemon trees will tolerate temperatures of about 35F without too much damage so if your garden doesn't freeze in the winter, your citrus trees should be okay outdoors.
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