As a broad generalization, one law department that has more experienced attorneys than another department will probably be more productive. One might say that young lawyers bring energy and new perspectives, but I would put my money on those longer in tooth who have developed experiential judgment and who have accumulated a store of knowledge about the history, people and workings of their company.
But how can we quantify experience as between law departments? Add up all the years that the lawyers in a department have been out of law school. Then, count an extra year for each year those lawyers have been in that law department. Divide the total by the number of lawyers and you have a snapshot figure that is weighted toward experience with the company (See my post of Nov. 28, 2005 on the average age of lawyers in a set of law departments.). If a lawyer came in an acquisition, count them as if they had been with the company the whole time. The legal-tenure metric as proposed does not take into account any life experience of lawyers before they became lawyers. Perhaps the way to do that is to give them a half-year credit for each year before they became a lawyer on the assumption that at least they have more maturity.
I have never seen a benchmark like this compiled, but it would be an interesting metric to correlate against total legal spending as a percentage of revenue or against lawyers per billion dollars of revenue.