I think he just meant with liquid water, some type of atmosphere, and approximately earth sized. Given this, my guess is that they find one within the next three years. If he meant "habitable" to human beings without protection, i.e. oxygen atmosphere etc., then this is extremely unlikely (less than 2% chance) that they will find such a thing by 2020.
Is it possible to have liquid water without life? I remember reading that an oxygen atmosphere was quite impossible, but am not sure about liquid water.
I would like to propose this as a thread for people to write in their predictions for the next year and the next decade, when practical with probabilities attached. I'll probably make some in the comments.