When you're using dried herbs, you can always buy them. However, they won't have the same potency as if you harvested and dried them yourself. When you dry your own herbs, you know exactly how old they are. You should never keep dried herbs and expect a high potency after six to eight months. Though, if you're cooking with them or looking for a mild effect, then it's okay to keep dried herbs for up to a year before throwing them out. When you dry your own herbs, make sure that you properly label everything, as many herbs will look the same after they're dried.