90% on a single language (German), and I'm comfortably beyond the "pizza order level" level of fluency, but probably still below a B1.

Trouble is, I meanwhile live in Germany and wouldn't be able to assign an exact figure to how much of my fluency is thanks to Duolingo, and how much is due to being exposed to the language in my daily life (comparatively little actually, as my job is in English, but far from zero).

If you are spending 5 minutes a day on DuoLingo - then you've spent 250hours on it, which I think might be tight to get to B1 standard anyway (but might be possible) - but you're probably spending those 5 minutes whilst you wouldn't be doing other language learning.