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Lettuce is a cool weather plant, so it is only good in the spring and late fall, or early winter if you have a good hothouse or other shelter to keep away the cold. Pick it before the weather gets hot.
I don't generally know when that is. I just harvest as needed, but take a taste before harvesting. Once it starts to taste bitter, the season is over.
The term is bolting...this means that the lettuce plant begins to send up a flower stalk from the center of the rosette of leaves. You will notice this difference in the center about the same time as the lettuce begins to have a white sap when you break off or cut the leaves.
There are several varieties of lettuce that are slow to bolt in heat.