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.