If you don't open the air-tight seal, can last quite long. Perhaps 2 years or 3 years tops? If opened, it's oxidised and exposed to moisture. I wouldn't keep that too long. Maybe can use it as flushing oil.
Lately there are tons of fake skunk2 manifold out there for sale in the marketplace, beware and don't be fooled. Skunk2 has posted this information on their website to warn...