It depends on the student's potential and the magister's plans. When a magister steps down or is... indisposed, his successor is usually named among his apprentices. There are tests, of course, but the position requires both politics and strength.

Some apprentices have been serving the same magister for fifty years. I've only served mine for five, but he is sure to retire soon and I am more than capable of passing the examination. [He clears his throat.] It also helps that I am the only apprentice he has.