Usually they die from frost or disease. Indeterminates, in a perfect world, would live forever, putting out new clusters along their vines as they grow. Determinates on the other hand, put out all their clusters pretty much over a short period of time, sometimes a couple weeks or a month or more (but not much more.)

Your plants may not be "spent" as you say, they just may have stopped putting out fruits due to high temps. If the plants are healthy, and the temps cooperate, you will get more output, until frost.

Mark

Mark is right on. ;) Assuming no disease problems and assuming adequate feeding over the season so far, the plants have just gone dormant as all tomato plants do during runs of high temps (check out the FAQ here about blossom dropping). Once the temps moderate, indet. plants kick back into production and keep going until frost.

If you haven't fed them recently most all benefit from a mid-season feeding about this time - food of your choice but keep it low nitrogen or you'll get a big green growth spurt but no fruits.

Dave

Or you can encourage them to keep growing. They'll grow even in hot weather and whenever it is cool enough they will set fruit (a little shade cloth helps). Too many people demand perfect conditions, and I sure don't get that. But I get fully ripe tomatoes. LOL!

5-6 hours of sunlight and heirlooms on 4 plants, and I am getting 2 rich ripe tomatoes per day.