Listen to the Rain

According to softlykarou's predecessor, you have a year or so before it really matters.

According to the Tokyo Embassy website, you have until the police pull you over and bust you for Driving While Gaijin.

Edit: The Japanese police say that you have a year, but that you can't keep leaving and getting new certificates to avoid taking the test. So you have one year at the maximum, and less if they don't like you.