The answer to 'when to treat for cockroach' is that you should treat for cockroaches as soon as you notice signs of an infestation, such as seeing live cockroaches, finding egg cases, or finding droppings. Cockroaches can multiply quickly, so it's important to address the problem early before it gets out of control.
The best time to treat for cockroaches is when you first notice signs of an infestation, as this gives you the best chance of eliminating the problem before it becomes more widespread. Cockroaches are most active at night, so you're more likely to see them in the evening hours. If you see cockroaches during the day, it's a sign of a larger infestation that needs to be addressed.