Sky High

Will Stronghold begins ninth grade at Sky High, a high school that exclusively teaches teenagers with superpowers. Will's parents are The Commander and Jetstream, some of the world's most famous superheroes. Will's best friend, Layla, who happens to have a crush on him, has the power to manipulate plant life. Will is anxious about attending Sky High, located on a floating campus reached by a flying school bus, because, unbeknownst to his parents, he has not developed any super powers. On the first day, he and the other ninth graders are harassed by a trio of bullies: Speed, a burly senior with super speed, Lash, a skinny senior with extreme flexibility, and Penny, a senior cheerleader who can clone herself. Because of his lack of powers, Will is slated to enter a curriculum for "Hero Support" and becomes a sidekick. His classmates include Ethan, who melts into a fluid; Zach, who glows in the dark; Magenta, who transforms into a guinea pig; and Layla, who joins the class in protest against the two-track nature of the school's education system. The class is taught by The Commander's former sidekick, "All American Boy."
The Commander is unaware that his son has been relegated to Hero Support, and shows Will his hidden trophy room. He is particularly proud of the mysterious weapon, "The Pacifier", which he took from his science-themed nemesis, Royal Pain, years ago. Unknown to either of them, Royal Pain, who had been presumed dead, watches the exchange from a hidden camera in one of the other trophies. As Will settles in to Sky High and makes friends with the other sidekicks, he comes into conflict with pyrokinetic student, Warren Peace, whose supervillain father had been imprisoned by The Commander. During a fight between the two, Will demonstrates super strength, impressing Gwen Grayson, a beautiful and popular "technopath" who controls machines with her mind. Will begins spending more time with Gwen and her clique of friends, ignoring the sidekicks and Layla, who reveals to Warren that she has loved Will for a long time. On the day before the dance, Gwen tricks Will into throwing a party at his house, and uses Speed to steal the Pacifier when she seduces Will into showing her the Secret Sanctum. After Gwen lies to Layla, who shows up to investigate the noise and believes the lie, Will breaks up with Gwen, refusing to attend the dance, even though his parents were invited as honored guests. Later, he looks through his father's old yearbook and sees a student who resembles Gwen. Believing that the student is Royal Pain and that Gwen is her daughter, he rushes to the dance.
At the dance party, Gwen reveals that she is actually Royal Pain. During her previous confrontation with the Commander, the Pacifier, which is meant to turn its target into an infant, had malfunctioned, turning her into a baby instead, thus faking her suspected death. She has since waited sixteen years for revenge. With the help of Speed, Lash, and Penny, she takes over the school and uses the Pacifier to turn the faculty and students into infants. After returning to school, Will apologizes to Layla, and teams up with Warren and the sidekicks to try to save the day. The sidekicks demonstrate their heroism after Royal Pain sabotages the school's anti-gravity drive and their powers come in handy restarting it. Will, meanwhile, discovers that he also has his mother's powers of flight when he is thrown off the edge of the school grounds and must prevent the campus from falling. Gwen and her henchmen are defeated and arrested and the faculty and students are returned to their proper ages. Will and Layla kiss, and a voiceover at the end reveals that they become a couple, he and Warren became best friends, and Ron Wilson gained superhuman powers after falling into a vat of toxic waste, thus becoming a superhero.

Question: Gwen has waited for revenge for how many years?
Answer: Sixteen years.