How many years did Rapunzel spend locked in the tower?

Rapunzel spent 18 years locked in the tower.

Mother Gothel abducts Rapunzel as a baby because the girl’s golden hair contains the healing power inherited from the Sundrop flower. Gothel hides her in an isolated tower and raises her there, visiting regularly while pretending to be Rapunzel’s protective mother. Rapunzel grows up without knowing that she is the lost princess of Corona.

As Rapunzel’s 18th birthday approaches, she becomes increasingly determined to leave the tower and see the floating lanterns that appear every year. Gothel refuses, insisting that the outside world is dangerous. Flynn Rider’s arrival finally gives Rapunzel a chance to escape and begin exploring the kingdom.

The number is tied directly to the film’s plot: Rapunzel leaves the tower on her 18th birthday, after spending essentially her entire life there. The lanterns are released annually on her birthday by the king and queen, who hope their lost daughter will one day return.