Planet of the Apes

Astronauts Taylor (Charlton Heston), Landon (Robert Gunner), Dodge (Jeff Burton) and Stewart are in deep hibernation when their spaceship crashes in a lake on an unknown planet after a long near-light speed voyage, during which, due to time dilation, the crew ages only 18 months. As the ship sinks, Taylor finds Stewart dead and her body desiccated. They throw an inflatable raft from the ship and climb down into it; before departing the ship, Taylor notes that the date is November 25, 3978, approximately two millennia after their departure in 1972. Once ashore, Dodge performs a soil test and pronounces the soil incapable of sustaining life.
After abandoning their raft, the astronauts set off through a desolate wasteland in hopes of finding food and water before their provisions run out. Eventually, they encounter plant life. They find an oasis at the edge of the desert and go swimming, ignoring eerie scarecrow-like figures around the edge of the water. While they are swimming, their clothes are stolen. Pursuing the thieves, the astronauts find their clothes torn to shreds, their supplies pillaged and the perpetrators â a group of humans that are apparently so primitive that they cannot even talk and dressed in torn clothes â raiding a cornfield. Taylor is attracted to one of the humans, whom he later names Nova (Linda Harrison).
Suddenly, armed and uniformed gorillas on horseback charge through the cornfield, brandishing firearms, snares, and nets. They capture some humans and kill the rest. In the chaos, Dodge is shot in the back and killed, Landon is wounded and rendered unconscious, and Taylor is shot in the throat and taken prisoner. The gorillas take Taylor to Ape City, where his life is saved after a blood transfusion administered by two chimpanzees, an animal psychologist Zira (Kim Hunter) and surgeon Galen (Wright King). While his throat wound is healing, he is unable to speak. Taylor discovers that the various apes, who can talk and are in control, are in a strict caste system: gorillas are police officers, military, hunters and workers; orangutans are administrators, politicians, lawyers and priests; and chimpanzees are intellectuals and scientists. The apes have developed a primitive society based on the beginnings of the human Industrial Era. They ride horses and have carts, rifles, and even primitive photography. Humans, who are believed by the apes to be unable to talk, are considered vermin and are hunted, either killed outright, enslaved, or used in scientific experiments.
Zira and her fiancÃ©, Cornelius (Roddy McDowall), an archaeologist, take an interest in Taylor, whom Zira calls "Bright Eyes". Taylor attempts to communicate by writing in the dirt, but Nova, who has been following him around, attempts to destroy his writing with her hands, presumably out of fear that his "intelligent" behavior will get him into trouble with the apes. The letters she doesn't destroy are then obliterated by Zira's and Cornelius's superior, an orangutan named Dr. Zaius (Maurice Evans). Back in his cage, Taylor steals Zira's pencil and notebook and uses them to write the message My name is Taylor. Zira and Cornelius become convinced that Taylor is intelligent, but upon learning of this, Dr. Zaius orders that Taylor be castrated. Before the castration can occur, Taylor escapes, and during his desperate flight through Ape City, passes through a museum, where he finds Dodge's stuffed and eyeless corpse on display. When Taylor is recaptured by gorillas, he overcomes his throat injury and roars, "Take your stinking paws off me, you damn dirty ape!"
A tribunal to determine Taylor's origins is convened by the president of the Assembly (James Whitmore), Dr. Zaius, and Maximus (Woodrow Parfrey). Dr. Honorious (James Daly) is the prosecutor. Taylor mentions his two comrades at this time. The court then produces Landon, who has been subjected to a lobotomy that has rendered him catatonic and unable to speak. After the tribunal, Dr. Zaius privately threatens to castrate and lobotomize Taylor if he does not tell the truth about where he came from. With help from Zira's socially rebellious nephew Lucius (Lou Wagner), Zira and Cornelius free Taylor and Nova and take them to the Forbidden Zone, a taboo region outside Ape City that has been ruled out of bounds for centuries by Ape Law. A year earlier, Cornelius led an expedition into the Forbidden Zone that found a cave containing artifacts of an earlier non-simian, and believed to be human, civilization. The group sets out for the cave to answer questions Taylor has about the evolution of the ape world and to prove he is not of that world.
Arriving at the cave, Cornelius is intercepted by Dr. Zaius and his soldiers. However Taylor holds them off, threatening to shoot them if he has to. Zaius agrees to enter the cave to disprove their theories and to avoid physical harm to Cornelius and Zira. Inside, Cornelius displays the remnants of a technologically advanced human society pre-dating simian history. Taylor identifies artifacts such as dentures, eyeglasses, a heart valve and, to the apes' astonishment, a talking children's doll. More soldiers appear and Lucius is overpowered, but Taylor again fends them off. Dr. Zaius is held hostage so Taylor can escape, but he admits to Taylor that he has always known that a human civilization existed long before apes ruled the planet and that "the Forbidden Zone was once a paradise, your breed made a desert of itâ¦ ages ago!" Taylor nonetheless thinks it best to search for answers, but Dr. Zaius warns him that he may not like what he finds. Once Taylor and Nova have ridden off, Dr. Zaius has the gorillas lay explosives to seal off the cave and destroy the remaining evidence of the human society. He then has Zira, Cornelius and Lucius charged with heresy.
Taylor and Nova, at last free, follow the shoreline and discover the beach-covered remains of the Statue of Liberty, revealing that this "alien" planet is actually Earth long after a nuclear war. Realizing what Dr. Zaius meant earlier, Taylor falls to his knees in despair and anger and condemns humanity for destroying the world.

Question: How much had the astronauts aged during their long space voyage?
Answer: 18 months