Answer: If the plants are healthy they just grow big. (Indeterminate plants will grow indefintiely until frost stops them.) Some gardeners will however train their tomatoes into two stems which they then tie onto a central stake. When the vine reaches the top of the stake they pinch out the top to limit the height. This method also requires a certain amount of care in limiting suckers and side shoots.
Based on my experience at home, as long as you are getting great tomatoes I wouldn't worry about pruning them.
Enjoy your crop!
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