It generally takes a few Formosan termites up to 10 years to establish a large colony. However, if the nest is already large, the termites can cause devastating damage to homes within a short time. The biggest problem in this regard is that you often don't know that they are present until they have already done substantial damage. These are insects which live in moist dark places and are not apt to show themselves to you. Protection against infestation is probably the best approach.