We've found it worked best to give volunteers a broad time range for the trials (i.e. drop by anytime between 5 and 9). The process is pretty intensive for the testers, and this way we could test a lot of people without making the volunteers wait around too long. We always provided food and lots of juice and non-alcoholic drinks (we didn't want any alcohol for fear of it blurring the trial results). We also tried as hard as we could to be as appreciative as we could of those being sprayed.