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http://www.gardenguides.com/95350-tomato-plant-life-cycle.html

 The tomato could be described as a short-lived perennial, but its span of life is somewhat variable. Under favorable conditions it will develop from starting seed to first ripe fruit in from 85 to 120 days of full sunshine with a constant day temperature of from 75 to 90° F., and with one from 15 to 20° F. lower at night. The plants will ordinarily continue in full fruit for about 50 to 60 days, after which they generally become so exhausted by excessive production of fruit and the effects of diseases to which they are usually subject that their root action and sap circulation become weaker and weaker until they die from starvation. From Philadelphia southward gardeners expect that spring set plants will thus exhaust themselves and die by late summer, and they sow seed in late spring or early summer for plants on which they depend for late summer and fall crops.

http://tomato-gardener.blogspot.com/2009/02/life-of-tomato-plant.html