Will Byers, one of the main characters in the hit Netflix series Stranger Things, was trapped in the dark and terrifying alternate dimension known as the Upside Down for a considerable amount of time. The exact length of his time in the Upside Down is not explicitly stated in the series, but we can estimate it based on the events that occurred.

Will’s disappearance in the first season of Stranger Things triggered a frantic search for the missing boy by his family and friends. However, it soon became clear that Will’s disappearance was no ordinary one. A mysterious creature, known as the Demogorgon, was revealed to be responsible for his abduction.
While Will’s friends and family were searching for him in the real world, Will was trapped in the Upside Down, a dark and twisted reflection of the real world. Will initially managed to evade the Demogorgon by hiding in a dilapidated castle. However, he was eventually caught by the monster, which took him to its lair in the Upside Down.
In the meantime, Will’s friends Mike, Dustin, and Lucas were working with Eleven, a girl with psychokinetic abilities, to find him. They eventually made contact with Will through a string of Christmas lights that he had set up in the Upside Down. Using these lights, Will communicated with his friends and gave them clues about his whereabouts.
Eventually, Joyce, Will’s mother, managed to make contact with him using a makeshift communication device made of Christmas lights and a wall. Through this device, Joyce learned that Will was alive and that she needed to rescue him from the Upside Down.
Joyce enlisted the help of Jim Hopper, the local police chief, and together they journeyed into the Upside Down to find Will. They eventually found him in the Demogorgon’s lair, where he was being kept alive as a host for the monster’s offspring.
The exact duration of Will’s time in the Upside Down is not clear, but it is estimated to be around a week or two. This is based on several factors, including the fact that the search for Will lasted for a week, the state of Will’s physical health and appearance when he was found, and the fact that he was able to survive in the Upside Down for an extended period.
During his time in the Upside Down, Will experienced several traumatic events, including being chased by the Demogorgon and being taken to its lair. He was also forced to hide and scavenge for food in order to survive. These experiences left a significant impact on Will’s mental and emotional well-being, which is explored in more detail in the subsequent seasons of the series.
In conclusion, while the exact duration of Will’s time in the Upside Down is not explicitly stated in Stranger Things, it is estimated to be around a week or two. His experiences in the Upside Down left a lasting impact on his mental and emotional well-being, and his journey to recover from his trauma is a significant part of his character arc throughout the series.