I've been using Duolingo to learn French for 1300+ days, and one of my ways to measure real improvement is by comparing how well I would do on the TEF ( Test d'Evaluation de Français). During the first two years, I noticed a clear improvement, mostly because of vocabulary, but Duolingo's interface and the way the classes are built made it harder for me to fully understand some of the subtlety of the language, so I feel like I'm stuck forever at a B1 level, even though I keep progressing on the "Duolingo Score".

I believe it is a great tool, especially if you want to skip the beginner's lessons, however, it seems to me to have diminishing returns with time.