The base oil has a very long shelf life, the shelf life of the additive package can vary a great deal though, it just depends on the exact oil, the container it's in, and how it's stored. Plastic containers breath, so some of the additives can escape, the longer it sits and the warmer it is, the more that may evaporate. Oil can also absorb moisture, this can cause it to break down and become acidic, so once opened it should be used up soon. Many companies say 4 years is reasonable life expectancy, But again, just because one exact oil may last 8 years when stored in perfect conditions doesn't mean another oil will.
There are also reactions that take place any time multiple chemicals are mixed together, so there are more additives mixed in to help stabilize the product and extend the shelf life. As these break down the properties of the oil can change, typically not to the good side. Some of these additives are in place to help keep all these different additives in suspension so they don't settle out, but it can oil work for so long, so the blend will eventually begin to separate, this is why you may see a thick layer at the bottom of the container.
Now if there is someone here that can simply look at a sample of old oil and say whether it's good or not they have far better skills than anyone I know. I can't see why anyone would use old oil when a gallon of fresh TCW 3 oil can be purchased for less than $20.00