Previous heads' tenures:
1. A. Prabhakar: ~5 years
2. R. Dugan: ~3 years
3. A. Tether: ~8 years
Historically, Prabhakar and Tether's long tenures in the position are unusual [1], with most directors staying anywhere from one to four years. This mirrors how DARPA works at the lower levels: the revolving door for DARPA Program Managers (and their programs) is very much real and 100% intentional: DARPA PMs are typically recruited from academia or industry, serve 3-5 year terms and then return to work in academia or industry. This keeps fresh talent and fresh ideas moving through the organization, and provides a sense of urgency and time-sensitivity to their work (successful projects get transitioned either commercially or into longer-term programs under other branches of the military-- they don't languish forever within DARPA by design).
[1] https://www.darpa.mil/attachments/DARPA_Directors_Sheet-web....
Too many orgs end up resting on their laurels - either getting too focused on research and not executing on technical advantages generated by that research, or too focused on execution and not enough into research.
Each DARPA project lasts two years (with possibility of extension). So a typical DARPA program manager will work in that role for just a couple years before returning to industry.
Just for example:
> The most prominent example of the Comcast/FCC revolving door is former FCC commissioner and current Comcast lobbyist Meredith Baker.
> Baker, whose views tended to side with the industry even before she went to the FCC, was appointed to to her FCC position in July 2009 and stayed there for nearly two years, cutting her four-year term short in June 2011 to move to Comcast as its senior vice president of government affairs.
https://www.opensecrets.org/news/2014/04/the-comcast-fcc-rev...