Strong winds and atmospheric turbulence hasten the breakup of vortices. Light winds may cause a vortex to remain or drift onto a runway and remain there considerably longer than normal. In the Billings scenario, note that the surface winds were very light. Surface wind was from 350, approach heading was 270. So, the Citation captain may have thought quartering headwind would push wake to south of his approach path.
Vortices move with the wind, so altering the flight path to the upwind side is good practice.
This can increase to 6 miles for small aircraft at the runway threshold. However, as previously mentioned, pilots assume responsibility under VFR conditions. A seemingly small aircraft, such as a Citation, Beech King Air, or Learjet, can generate dangerous wake for a similar-sized or lighter aircraft. There are several such accidents involving two small aircraft that have occurred in the past five years. For example, a Piper PA-18 was upset following a Piper Cherokee Six on takeoff.