I have been told in some states they are only sold to licenced exterminators. One may work better than another in some parts of the country or with different kinds of soil. I'm not sure whether this is true or an old exterminators' tale. I selected mine by asking Terminix and Orkin what they use. From what I've been able to learn, inaccessible areas, like under the slab, are not likely to have their protection disrupted, so if they are good for a year, they'll probably continue to hold for the life of the stuff.
Areas exposed to gardening, rain, sunlight, animals, etc are less secure but easy to self-treat.
I check the slab for mud tubes quarterly. I then watch the area for a week or so. The hornet spray usually does it. If another tube appears I trench about 10 feet on either side of breach and apply the poison according to the package instructions. I have never seen them come back to that area again.