[sharker8]

Followers of Krashen know that it's sensible input that results in language acquisition. Duolingo gamifies the frequency of sensible input. Therefore Duolingo will work if frequently and consistently used. Fluent is an outdated term. It probably takes about six months to achieve any lasting result. Illusion of progress is impossible to prove or disprove probably.

[veridies]

I just want to note (this is what I studied for my MA) that Krashen is very controversial in the field. Many researchers argue that output is critical to learning, and "interactionism," the idea that learning is facilitated through conversation, is probably the dominant theoretical approach at the moment. See, for example, Second Language Learning Theories, by Mitchell, Myles, & Marsden, chapter 6.

[dplgk]

Six months of what to achieve what result?