In a conversation with Collider, the Duffer Brothers (Matt and Ross Duffer, the creators of the series) admit that it was longer than planned, at the same time as they reveal that the fifth and final season will not be as long. Matt Duffer explains:

“We thought Season 4 was going to be eight [episodes], and they were going to be regular length. So if you had interviewed us before four, that’s what I would’ve said. I think we’re aiming for eight again. We do not want it to be 13 hours. We’m aiming for more like 10 hours or something. I think it’s going to be longer than Season 1 because we just have so much to wrap up, but I do not think it’s going to be as long as Season 4. “

Ross Duffer then developed why we can expect things to go faster next time:

“This season, for instance, it was two hours before our characters even realized the monster was killing people in Hawkins. They know what the threat is now, and so that will help speed it up.”