Given warm weather and abundant rainfall, organically-grown tomatoes ripen in 60 to 85 days from the time seedlings are set out. When the fruits begin to turn red, check the plants every day and pick those tomatoes that are fully red (or yellow, if you are growing a yellow variety) and are firm but not hard. Overripe tomatoes will fall off and rot quickly.
A very light frost will usually kill a few leaves, but the plant itself will continue to grow and produce. However, anything more severe than a touch of frost is likely to kill the entire plant. If you hear that frost is coming, protect each plant by draping it in plastic sheeting or old bed sheets; or you can pull up each plant by its roots and hang it upside down in your cellar until the fruit ripens.
Neither method is guaranteed to work, and in cool areas an early frost almost always means the end of tomato harvesting.
Unripened tomatoes need not go to waste, however. Pick them and ripen them quickly in a warm place, or wrap them individually in newspaper, and store in a cool, dark place for slower natural ripening. Also, check your cookbooks for ways to use green tomatoes.