The effects of a single dose of catnip, either in a spray or as a fresh or dried herb, will generally only last about 10 to 15 minutes. Its impact also decreases over time if given to your cat often so consider offering it no more than a few times a week. Ingesting catnip in treats or as the fresh or dried herb can actually be healthy for your cat’s digestive tract. But the effects will be less wild and will instead cause your cat to be mellow for longer because the natural oils are released, absorbed and metabolized more slowly. Those essential oils can degrade over time, especially when exposed to the open air. So store your catnip in tightly sealed containers in the refrigerator or freezer. Consider using it up or replacing it annually to ensure maximum freshness.