Disclaimer: opinions ahead!
Many years ago, my time quitting was with vir. Couldn't beat him. Ran out of ammo. He hit hard when I got close. Tried with a squad. We died. Tried alone again, no good. Friends dragged me back a couple of years later and I'm still here.
So, first station for quitting: when you have to deal with game systems that you have no knowledge about. When does that happen? When you actually need to mod your weapon and frames. Very. Early.
Since I'm on the subject - there is nothing in the game that explain what's going on in the plains, stuff like what's the quills and where to find them. The map doesn't show us all the npcs and it's impossible to guess something like the quills even exist. Warframe does that with a lot of things. Kuva, focus, mods... Crafting weapons is mid way because by talking to the vendors we can guess what we're suppose to do. I remember that when I started I had no idea that the mods station is a thing I can interact with. This is the reason for why I'm investing time in explaining things to newcomers. I had 30-45 minutes of going back and forth with q&a just because new people can't guess all of this stuff. More than once. Now gladly there's a lot of content explained on youtube and info is easier to find, but still.
To summarize - the start of the game right when you finish earth and ask yourself if you even do something with the things you collected, and then there's nothing in the game itself to give you that answer.
Second point of quitting - when the game focus shifts from exploration to grinding.
After doing all the quests, fighting all the bosses and completing all the missions. When someone realize they have the game in their pocket the sensation of repetition becomes stale. The gameplay is awesome and fun but for some people loosing the experience of finding out more about warframe is too much. They want something that will keep exciting them. Doing the same mission with different weapon or warframe is good and fun for the first few minutes, after we've done just that for many many times. I love testing out new warframes and this bothers me less, but that's just me. I think people quit at some point here.