According to Foreign Language Instruction in
U.S. Schools: Results of a National
Survey of Elementary and
Secondary Schools, which was published by The American Council on the Teaching of Foreign Languages in 2011 (emphasis mine),
At the secondary school level, an estimated
10.5 million students out of 25.7
million (41%) were enrolled in language
I came across this interview by The Economist with Timothy Doner, who was a teenage polyglot (the interview is from 2013, when he was 17 years old) that had "learned" 20 languages in four years. That would translate into about 2.5 months per language (which he mentions in the interview that sometimes he only studies a language for a couple of months). ...