Deutsche Welle suggests A1 is reached with about 75 hours of German tuition, A2.1 with about 150 hours, A2.2 with about 225 hours, B1.1 with about 300 hours, and B1.2 with about 400 hours.[5]
Cambridge ESOL said that each level is reached with the following guided learning hours: A2, 180–200; B1, 350–400; B2, 500–600; C1, 700–800, and C2, 1,000–1,200.[6]
Alliance Française has stated students can expect to reach CEFR levels after the following cumulative hours of instruction: A1 60–100, A2 160–200, B1 360–400, B2 560–650, C1 810–950, C2 1060–1200.[7]
(https://en.wikipedia.org/wiki/Common_European_Framework_of_R...)
For those who don't know about CEFR, C1 is basically fluent. It might take a bit longer to learn Mandarin, Russian or Arabic, but it's just by a reasonably small constant factor (based on FSI numbers that I can't find right now).
They key is to make the practice good practice, and not just mindless repetition.
This - it needs to be emphasized more. I feel a bit pedantic when I make this point, but I really think it's crucial.
If you read the original research, it's not 10000 hours of just practice. There is a reason many people can play golf for 30 years (and probably accumulate 10000+ hours of play), but still never even get close to shooting par. The term they use in the research is deliberate practice. The quality of the practice is just as important as the practice itself. Practice does not make perfect, practice makes things a habit. If you develop the wrong techniques in an area, you'll reach plateaus and hit a point where you can't progress. Deliberate practice requires tasks that are designed to stretch you in specific areas and have fast feedback loops so you are able to correct mistakes quickly. This is easier to apply in some domains (music, sports, etc...) than others.
One amazing thing is just how that practice translated into so little time in actual games. I.e. if a football team plays 20 games a year (1 hour each), and the offense is only out there half the time, then a wide receiver is playing for at most 10 hours a year. Jerry Rice played for 20 years, so that works out to 200 hours of actual game time over his entire career!
In each case, I've been able to have conversations with native speakers after the program ended.
The next step, to really master any of them, would be going to one of those countries. Suppose I spend four hours a day in conversation with native speakers. 1 week = 28 hours. 3 months = 336 hours.
Total of 381 hours. I'd be astonished if that program didn't make someone fluent, at least in a language with some similarities to their native language, as close as any of the romance languages are to English.
I'm speaking from experience, having got part of the way with Italian and Portuguese, and having had a six month visit to Cuba make me fluent for all intents and purposes (but I didn't do Pimsleur before, just ineffective college classes)
The key to shortening is the intro lessons which make all additional practice much more effective, since you can simply practice by fluid conversation. This isn't the only method; I've heard very good things about Benny Lewis' method as well.