Essential oils are essential oils and you can keep them for ever right?
Well, not quite. Many oils deteriorate over time, especially once opened. In particular the citrus oils have a relatively short shelf life once air which allows some components of the oil to oxidise gets in.
Unopened, this is less of a problem and if along with them being unopened, they are kept cool, ideally in the fridge, they will last far longer than I ever need them to. Years rather than months.
A few oils such as patchouli actually improve with keeping but this is very much the exception.
The answer is to not buy oils in larger quantities than you can use in a few months and to keep them in a cool place, especially once opened. This will slow down the rate of oxidation.
In my massage and aromatherapy practice in Cambridge, I have between 50 and 60 oil bottles open at any one time but am careful to keep them cool when not in use and buy only small quantities of most oils at a time. The exceptions being Geranium and Frankincense that I use in making handcream using organic olive oils and beeswax. The beeswax has the effect of slowing down the oxidation of the essential oils in the mix which will last for 12-18 months, longer if kept in fridge between use though this makes it a little hard so it needs warming first. The limiting factor is in fact the olive oil which will oxidise and go rancid if left too long or kept in too warm a place once opened. There are other oils I could use which would last longer but certainly with some of the alternatives, the product would not be quite as nice to use.
Commercially, many companies add artificial stabilisers to products such as handcream in order to prolong the shelf life. I prefer to make small batches, little and often.