Answered for language learning in general. No specific language was set for this question, so the findings are drawn from general second-language research rather than one language’s literature.
Based on the available research, immersion by itself is not consistently sufficient for fluency — how much someone gains from immersion varies enormously between individuals, and several studies found that structure, feedback, or explicit attention to language form made a bigger difference than raw exposure time. Immersion clearly helps (especially for skills like reading speed, vocabulary, and overall exposure to natural language), but multiple sources show gains are uneven across skills, some learners barely benefit from exposure alone, and deliberate teaching or corrective feedback often accelerates or even matches what immersion produces on its own.
A longitudinal brain-imaging study of adults living in the UK found that simply spending more time immersed in a second language did not reliably predict higher proficiency scores over a three-year period — the correlation between immersion time and proficiency was weak and disappeared entirely by the study's second measurement.6
A university study comparing 'semi-immersion' (some English-taught courses) to 'full immersion' (all courses in English) found that only the semi-immersion group showed statistically significant grammar gains after a year; full immersion alone showed only a non-significant trend. The authors concluded that adult learners likely need some deliberate focus on language form, not just exposure, to see clear gains.7
Studies of French and Japanese immersion classrooms found that simple exposure to the target language in class was not enough on its own — the type of correction teachers gave (subtle rephrasing versus prompts that push students to self-correct) strongly affected whether students actually fixed their errors, and this varied by the type of error and by classroom context.4,5
Long-term study of bilingual children showed that immersion-style schooling produced children who read about as well as monolinguals but had noticeably weaker vocabulary in each individual language, with other skills landing in between. Being 'immersed' didn't lead to uniformly native-like ability across all areas of language.3
An experiment using an artificial language found that people taught through explicit, classroom-style rule instruction reached the same overall proficiency level as those taught through implicit, immersion-like exposure, though their brains processed grammar differently. This suggests immersion is not the only path to a given proficiency level.1
A large study of children's informal English exposure outside the classroom found substantial language gains from exposure like gaming, social media, and speaking practice, but also very large differences between individual children in how much they gained — meaning informal 'immersion-like' exposure alone doesn't produce consistent results for everyone.9
A study of an English-medium university program found that lower-proficiency students needed targeted language support to succeed, since language proficiency and academic language skill (not immersion exposure or motivation alone) were the strongest predictors of course success.10
A 7-week program of extensive reading (reading many easy books) raised Japanese high school students' English reading speed by about 30%, even though students read only a handful of books on average, suggesting that structured, sustained exposure to input can build fluency-related skills without full immersion.2
Don't rely on immersion alone if your goal is well-rounded fluency — pair it with deliberate practice. Specifically: (1) seek out corrective feedback (a tutor, teacher, or language partner who corrects you) rather than assuming errors will self-correct through exposure, since studies of immersion classrooms found the type of feedback mattered enormously for whether students fixed mistakes; (2) if you're in a full-immersion setting (studying or living abroad) but progress feels slow, add some explicit grammar or form-focused study, since research on university immersion programs found this made the difference between measurable gains and none; (3) don't assume immersion will fix vocabulary gaps — study shows immersion learners often lag in vocabulary even when other skills catch up, so targeted vocabulary work (e.g. flashcards or reading extensively) is worth doing alongside exposure; (4) if exposure is passive (just being around the language), make it interactive — studies found interactive exposure (gaming, social media use, conversation) produced better results than passive exposure; (5) expect big individual variation — some learners get much more out of the same immersion experience than others, so track your own progress rather than assuming time-in-immersion alone will do the job.