On a tree, it usually takes between six and nine months for lemons to ripen, depending on the conditions during those months. Warm temperatures and lots of water are key factors in helping the fruit ripen more quickly.
Lemon trees can provide ripe fruit throughout the year, so it is important to recognize the signs of a ripe lemon. Lemons are typically between 2 and 3 inches in size when they are fully mature, and they may still have a greenish-yellow color. Wait to pick lemons until they are completely yellow and have a glossy finish across the surface of the peel.