Not very far. I have seen people with excellent memory. Quite often, once they see a new word, or hear a new word, they will remember it for as long as a day. That's all there is. If they don't review, or practice in one form or another, they will forget what they have learned.
It's a great advantage for people who have excellent memory, but also a great pit fall. This type of people tend to be over confident and neglect the fact that learning a language requires more than just memory. Learning a language is a journey with commitment.
