Storage:
Storage is important. The oils should be kept in brown or dark-coloured bottles away from light, heat and the damp. Keep the tops tightley closed when not in use^[1].

Nearly all vegetable and essential oils oxidise, or go rancid.  The result is an unpleasant odour in vegetable oils, and a distinct loss of freshness and worsening of odour in pure essential oils.  Eventually, the oils become cloudy, and their therapeutic value is considerably reduced.  Once oxidation has set in, it is a process that cannot be reversed.  This is why it is important to keep oils in air-tight containers, preferably well filled with minimal air at the top, and never to put fresh oil into a used bottle^[4].

Take care not to contact the bottles dripulator to your skin as this will decrease the shelf life of your oils. The natural bacteria on your skin can contaminate your oil, and begin the process of decomposition. Discontinue use if the blend smells rancid.  Just as you can tell if milk or butter has "gone off" by its odour, the same applies to essential oils.

Shelf life:
The therapeutic life of pure essential oils is about two years, although some would argue that they last longer than this. Certainly their antibiotic and other properties can still be utilised in non-body methods such as air-fresheners, kitchen surface wipes, perfumes or celebratory and gift purposes, when merely their aroma is important^[1].

Unfortunately, essential oils are not yet dated when sold, so it is impossible to tell how old they are when you buy them - another reason for going to a reputable supplier^[1].

Massage blends have a shorter shelf life than pure essential oils, as they contain less of the anti-bacterial components of essential oils. However, I personally have had a blend containing Chamomile, Geranium and Lavender in a Sweet Almond oil carrier, that lasted greater than two years before I noticed a slight rancidity to the smell of the blend.  

Advice:

Wheatgerm can be added to your essential oil massage blend to help improve it's shelf-life.  The anti-oxidant properties of wheatgerm oil will help prevent oxidization and rancidity.  Wheatgerm is a nourishing oil containing Vitamin E, and is good for mature skin and dry-skin conditions^[2].  Although it is too rich and heavy to use on its own as a carrier oil, adding a few drops (up to 10% composition) to any essential oil blend will improve the blend properties on many levels, including penetration of the skin and extending its shelf-life^[3].  Your oils will not last forever, but will keep longer than usual; exactly how long depends on how often the bottle is opened, but five months is the average period, if the oils are fresh to begin with^[4].




References:
^[1]  Valerie Ann Worwood (1990). The fragrant pharmacy: A complete guide to Aromatherapy and Essential Oils.  Bantam Books, London, UK.