The answer to 'when to treat for extermination' is to treat for pest infestations as soon as you notice signs of a problem. Catching pest issues early is key to effective and efficient extermination. Common signs of an infestation include seeing the pests themselves, finding droppings or nesting materials, or noticing damage to your home or belongings.
It's best to treat for pests at the first sign of a problem, rather than waiting until the infestation has become severe. This allows the extermination treatment to be more targeted and successful in eliminating the pests. Waiting too long can allow the population to grow and spread, making it much more difficult to get rid of them.