Will is in the Upside Down for about one week in Season 1.

Timeline in the show

  • Will disappears on the night of November 6, 1983 , when the Demogorgon takes him from his shed into the Upside Down.
  • Over the following week , Joyce, Hopper, and the kids search for him in Hawkins while he stays trapped in the Upside Down.
  • Fans often point out that the town is already holding a funeral for him after roughly a week, which lines up with dialogue and forum discussions about the timeline.

In-universe interpretation

  • In story terms, Will survives about a week by hiding, using his fort (Castle Byers), and later being kept alive by the tendril in his mouth, even though the Upside Down’s atmosphere is toxic and hostile.
  • Some discussions suggest that time in the Upside Down may feel distorted, but the canonical calendar in Season 1 still supports a span of roughly one week in real-world time.

So if you’re writing, theorizing, or arguing on a forum about how long was Will in the Upside Down in Season 1 , the best-supported answer is: about one week in Hawkins time.

TL;DR: Will is trapped in the Upside Down for roughly seven days between his disappearance on November 6 and his rescue near the end of Season 1.

Information gathered from public forums or data available on the internet and portrayed here.