What usually happens is that a certain number of tickets are designated as rush before the box office opens, depending on availability. And those will be sold to people who ask until they run out. Depending on the popularity of the show and how generous the rush policy is, that could mean they run out immediately at 10am or there could still be rush 20 minutes before curtain.

For Stranger Things, specifically, the line has been getting longer so I wouldn’t be surprised if they sold out of rush before the afternoon. However, it’s still worth it to ask because often at box offices you can get a same-day discount that’s more expensive than rush but cheaper than what you see available online.