As far as Green Pastures can tell, fermented cod liver oil may never expire. In fact, there is quite a bit of evidence to show that it even gets better with age! The oil is protected from oxidation by natural components of the oil and so it is protected from oxidative rancidity much more than other cod liver oils. You can read about some of the protection methods here, but to fully understand it, much more research is needed. Suffice to say fermented cod liver oil has quite a long expiration date. It is very different to other cod liver oils that are commercially produced, and has a great variety of micro-nutrients which protect the oil. This being said, I’d still recommend you keep the oil in the fridge once opened.
To give assurance to customers about how fresh their products were, Green Pastures used to only apply a manufacturing date (MFG) to their products, to indicate how fresh it was. For a long time, Green Pastures did not add an expiration date to their product.
Many people feel quite uncomfortable about the infinite expiration date though, especially when little is really known about how it works. To remedy this, Green Pastures stopped adding the MFG date, and instead added an expiration date, which is 2 years from the date of manufacture. Green Pastures have extensively tested their oil, and can very confidently say that it is safe at the 2 year mark. Beyond this point, it still appears to be stable, although research beyond this point is not as abundant.
It is worth noting that even if your oil is well within the expiration date, it is best to keep the oil in a fridge when you open it, and try and consume it within 2 months. Although it is a very stable oil, there is no point in risking oxidative damage – it is still a natural and whole food, and damage is always possible to the nutrients.