When magic became formally 'governed' by the Towers, representatives from each formed a Council to better maintain information and unify all practitioners. Currently there are three Towers, and three Orders, each representing a general faction of like-minded mages.
Positions within the Council are voted on every seven years, and all mages who have formally passed their Tests can vote and hold position within their Order. This vote determines the head of each Order [ie Master of the White, Master of the Black] as well as the 'Head of the Order' and other positions.
Such votes are generally held at the current Head of the Order's Tower, wherein any changes of position are heard, as well as any serious business amongst the magi at large, and some of the more interesting or highly anticipated Tests are played out. Gatherings are generally held each new moon, with position changes [such as the next Council meeting] then put into effect by the end of the current lunar phase.
Though some terms last longer than others, stretches unimagined by human diplomats are common with the Trimagus. Demetrius Wanlorn, the current Head of the Order, has held that position for some 60 years, since assuming power from the insane Virgil of the Black near the end of World War II. Because of their longevity, such things are generally regarded from the long-view, and some secretly-rumored positions can be held for life…
Many supernatural Bloodlines also grant mystical abilities, and many of these gifted offspring show magical aptitudes. Though a minority of a minority, most of at least 1/2 blood are welcome within the ranks of the Trimagus, as any of pure blood would be a supernatural creature onto itself.