I'd bet that the time spent is being underestimated. Duolingo doesn't track time and works in "lessons" containing a series of something like 15 questions each. You can do one of those in less than 10 minutes, but if you do two, you're over 10 minutes. There's no option to do less than a lesson (even "practice" is arranged as if it were a lesson), and bailing in the middle wouldn't be recorded as anything.
That could well be the case. But I suspect that there's also a great deal of variability in what people mean when they say "conversationally fluent". For example, I completed the entire French tree in Duolingo a few years back, and would consider myself nowhere near "conversationally fluent".
