The shelf lives of our essential oils can vary by product. For example, some oils and blends are best when fresh. Others may improve with age or may be unaffected by their age entirely. Though it varies across the board, it is important to note the factors that are a surefire way to age any oil quickly: oxygen, light and heat exposure. Generous exposure to any one of these elements may speed up the natural aging process.
Highly-regarded therapists have said that even citrus oils, if kept in a closed bottle, with very little "air space" (meaning the bottle is full), in a dark, cool or even cold (as in the refrigerator) can keep for 2 years or more without loosing potency.
While we store our essential oils under such conditions as well as receive those from the most recent harvest when this is optimal, you can extend the life of your oils by keeping them in a cool, dark location, tightly closed. If you buy a large quantity, you might consider buying one or more smaller empty bottles which you fill with your "stock bottle," and keep the stock bottle in cool storage while regularly using the essential oils from the smaller bottles and not needing to be concerned about their storage conditions.
IN GENERAL, citrus oils have the shortest "shelf-life," followed by herbs and floral oils, then woods and resins. This is very general, and truthfully, as long as you take care of your essential oils by storing them properly, it's not likely you'll need to concern yourself about them loosing potency.