Hi,
Some essential oils develop in a blend and their shelf life can be approx. 18 months. Some can last even longer. I always find that the resinous oils develop with age, i.e. frankincense and myrrh. I have a patchouli that is pretty old that still smells wonderful and is way past its Best Before Date.
I find the citrus oils do go more quickly - you can usually tell when an oil goes off as its colour changes quite dramatically.
Something to be born in mind is not to let too much air in, so decant your oils if bottle is partially full so that its in a smaller container with less air.
Luv Binah