I don't believe in "finished." TDD can be used as a way to strive for perfection, but that isn't its most effective use. If you have a big system, then the parts that you touch all the time should be absolutely rock solid, so you can make daily changes confidently. As you drift out to the periphery of the system, to parts that don't change often, the tests can be spottier and the design uglier without interfering with your confidence.
