How long did Medusa live?

In order to offer a date of her demise, she would have had to exist, and she didn't. The story of Medusa is one that is told to offer as allegory to the concept of man overcoming adversity, fear and the wilderness. The monsters the founder-king heroes slayed were all indicative of darkness and powers of nature. Medusa, for example, represented the dangers of the sea (since she was connected to Poseidon).