Temple weddings are more accurately called temple sealings, since the prophet Elijah committed the sealing keys to Joseph Smith on April 3, 1836 (see Doctrine and Covenants 110:11-16). Because those priesthood keys were restored to the earth, our families can be sealed together for eternity.
The sealing ordinance itself is short. The couple kneels at an altar in a special sealing room. The words of the ordinance, like the sacrament prayer, are already written: the sealer (the temple worker who performs the sealing) asks the couple if they take each other as husband and wife. Then he seals them to one another and pronounces blessings upon them. He may also add a few brief words of counsel.
The ceremony may not last very long, but the sealing can last for eternity.