Editor's note: This is the first in a series examining how executive-level positions at law firms are changing to fit the industry.

The longevity of chief marketing officers can swing wildly among large Pennsylvania-based law firms. But as more firms seek to build their marketing leadership from the ground up, observers say the revolving door may be slowing.

Just six of Pennsylvania's 11 Am Law 100 firms have held onto the same CMO for more than three years. Three of them—Ballard Spahr, Reed Smith and Morgan, Lewis & Bockius—saw the position change hands in the last year. Drinker Biddle & Reath has yet to fill a CMO vacancy.