There was no set number of years that a priest could serve as the high priest. He served as long as he lived. See Talmud, Yoma 9a.

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And Annas the high priest, and Caiaphas, and John, and Alexander, and as many as were of the kindred of the high priest, were gathered together at Jerusalem.

The role of a high priest in casting spells is that the priest can cast more powerful spells in which a normal spell caster cannot complete. The high priest specializes in love spells.

He was the high priest called Caiaphas.

The high priest was in charge of running the Temple and at one point was the co-head of the sanhedrin.