In general, a B&B agrees to serve at least a two-year term. They serve until they decide to step down. Likewise, the Crown, at their decision and at any time, may ask a B&B to step down.
The procedure to select a new B&B varies over time. The detail of the selection process is handled by the current Kingdom Seneschal and Crown. Sometimes there is unofficial nonbinding polling of the populace to see who the Barony would prefer. However, a Crown can choose anyone they see who they believe will maintain, protect, and grow the barony. As mentioned earlier, a B&B could sit for many years and see many Crowns.
A Vicar and/or Vicaress are the temporary representatives of the Crown of Ansteorra who will sit in lieu of a Baron or Baroness. Also appointed by the Crown; their length of service is at the discretion of the Crown. They have most of the powers and obligations of a sitting landed B&B.
Invested at War of the Rams, 2018. Their Excellencies are the representatives of the Crown to this populace.