Freeze-dried instant coffee properly vacuum sealed in it's original jar can last decades. I read of an account of a guy who found a jar that was over 20 years old and he claimed it tasted like new. Years ago, my mother-in-law made me a cup of instant coffee (she had no other coffee in the house), and it tasted like typical instant. But I later found out that the jar she used has been opened for over 5 years. Being a non-coffee drinker, she just had it around for guests.
"There is no hell. There is only France"- Frank Zappa