If you are using all the right tools to fight against roach infestation such as glue traps and DE powder you will soon notice improvement. This is one of the reasons why roach motels are so versatile and a good option when you want to assess the severity of the infestation.
If you notice that the traps are not as full, there are fewer droppings and skeletons all around you are on the right path to a pest-free home.
At the end of the day when the roaches are eliminated the traps will remain empty for a long time. But, even if you managed to kill all the bugs you should leave a few roach box traps out and check on them from time to time to ensure no bugs are coming back.