Does anyone have any thoughts on how long a judicial clerkship should last? One year, two, five? Does it make any difference?

Clerkships are meant to provide you with the experience. They are generally not meant to be made into a career. Even though many people do turn a clerkship into a career, its not a bad way to draw a paycheck as a lawyer. But, if you are using it as a bridge between law school and private practice, generally the longest a clerkship should last is two years, too much more than that leave a too large a gap between law school and private practice. But, even that depends on the type of clerkship you have, a court of appeals clerkship could last years and in the end makes you a very valuable commodity. A district court clerkship, in my opinion, has little value other than experience, it does not make you as marketable as a court of appeals clerkship. So you do need to evaluate certain factors depending on your situation and goals.