Walker had been acting director of DARPA since January 2017 (eventually formally becoming director in November 2017), so, when his resignation takes effect, he'll have been in the position for three years-- actually a few months longer than Dugan.

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....