There is usually an expiration date printed on each stabil bottle.
And, their website FAQ has this to say about the matter:
Q: What is the shelf life of STA-BIL Fuel Stabilizer?
A: 2 years after the bottle has been opened, provided it has been tightly capped and stored in a cool, dry place. If the bottle is never opened and under the original factory seal, than the shelf life is 5 to 6 years from manufacture.