I've been buying extra virgin olive oil in a 16.9 ounce (dark green) bottle and it takes me about 6 to 7 weeks to use one bottle. (I like buying it at Aldi's because it's only about $3.99.)
How long does it take for the oil's antioxidants to become oxidized and destroyed?
The best information I could find online is in the following link. It gives praise to extra virgin olive oil (saying that it resists oxidation) but doesn't say how long it will keep without becoming overly oxidized.
Oxidation of Fatty Acids - Olive Oil Stability - Why Olive Oil?
If you scroll down near the bottom you'll see a negative comment by Michael Bradley. He claims not all olive oils labeled "extra virgin" are extra virgin, even in Europe (Spain, Italy and elsewhere). If that's the case, what are the chances that I'm really getting "extra virgin" olive oil for only $3.99? Probably slim. Also, he says that very few companies provide the harvest date on the label. So if the oil is over a certain age, like 16 months, it can become seriously declined in quality.
He says they are working on publishing a grading system. Until that time comes, I won't be buying any more olive oil, or any oil for that matter.