Technically essential oils should never go off. They consist of many natural chemical constituents, some of which are plant alcohols, (terpenes and hydrocarbons etc) which are natural components within the oil. When stored properly, in less than 30°C, out of direct sunlight, your essential oils should last you for many years. If a seller of oils advises you to routinely store your essential oils in the refrigerator, you may wish to reassess the quality of the product. Best practice related to quality control will see essential oils batched and expiry dated. This is done in order to trace product, and facilitate company guarantees of quality standards within a measurable time frame. However the life of quality essential oils will extend beyond this time for years.