From what I've read (and it's been a lot)
e-liquid when stored in a cool, place, out of sunlight, stays for 24 months, tbh I wouldn't let it sit that long.
Many e-liquid bottles have expiration dates on them.
From what I can gather the liquid will lose potency and flavor from oxidation when kept to long.
P.S. one way you can tell it's too old is if the liquid seems to be separating into different layers and doesn't come back together when shaken.