Anyone with a security clearance is subject to a polygraph exam at any time. Depending upon the sensitivity of what you do, it can typically be never, seven years, five years, or yearly. Each exam may consist of multiple sessions.

(I was cleared to a bunch of different stuff over 40 years, and I did not get my first polygraph until 30 years ago (about 13 years in). I was getting annual polygraphs for five years, then got them every five years for the remainder of my career.)

My circumstances were not typical, but five years is a typical cycle. It coincides with the "periodic re-investigation" interval, which can also be different, depending upon the nature of the clearances.