I was a long time duo user (1000+ day streak) learning german. I wanted to be able to read german (books etc) rather than being able to converse in german. The streak system and the gold->diamond tier system kept me going but after a while I realized I had plateaued and I felt like I was forgetting more than I was learning.

Then I realized, I had got to the point where I was good at answering what duo wanted to get the daily requirements rather than learning.

One day I just stopped and let it go