I have a strong suspicion that Duffers are secretly aware of how many seasons there are left. And it does not mean that they necesserly know how much thay want to write, but how much they have to because of Netflix executives.
I recently was reading a David Harbour interview that was between Season 1 and 2 and in one of the responses he told that there was no news about season 2. Later he responded to a question on what would he like Hopper to become in season 2, he said that he would like to see him as a surrogate father for Eleven.
Plus, in one of the interviews Duffers mentioned that they were green lit for season 2, almost right after the premier of Season 1. Moreover, Millie also accidentally revealed on one of the ComiCon panels with Harbour that she had difficulty remembering the lines, since they jumped on season 2 filming, right after the Season 1. I believe it was from the same person who had previously spoken about the Duffers and NetFlix having different ideas for how long many seasons should go. Obviously we can't take that as anything official yet, but the rumor is out there.
From this I think that you can deduce that Duffers knows how many seasons left. While viewers are left in a state of limbo, all for the sake of marketing and hype. I think Netflix ia going to push and prevail and we are going to have Season 5. Moreover, I dont see how Duffers can continue writing if they don't know the overarching story and it's finally.
Recently David Harbour said that he and Duffers were discussing overall storyline, even back at season 1, and he suspects that he knows how it ends. So overall, it is pretty clear that the show will soon be over.
The writers probably have a rough outline of where they want the series to go up until the end.
Most shows do.
However that doesn't mean they know exactly what's going to happen in-between then. They've got to fill in the gaps and they might not know exactly how many episodes they need right away. There's no way to know unless they formally announce it.
And things may not be set in stone, especially considering they still write scripts for the back half of the season while filming. Plans change and they might realize they have less or more story than they anticipated.
So it only makes sense to go for as long as you can produce quality (Or money).