Top Law Schools

How long does it normally take to complete a JD/PHD at the top law schools (Yale, Harvard, Stanford etc.)? Harvard lists a plan for both five and six years, and suggests students may also outline their own plans. But it's five to six years, not counting your dissertation. How long would it normally take for one to complete his/her dissertation? (I know it can vary, so what would be the ball park figure?)

Moreover, does anyone know if top law schools give students the option of school in the summer? Do graduate students have school in the summer? If so, can they use this to finish their program faster?

Here is an unrelated question:

Do law schools give any kind of preference to students interested in pursuing dual degrees (i.e JD/PhD)? If two applicants have identical numbers, wouldn't the admissions folks want to admit the student who will enroll in the additional degree program (and therefore pay the university extra tuition)?