The Foreign Service Institute (FSI) has created a list (based on their experience training US diplomats) to show the approximate time needed to learn a specific language as an English speaker.
This is the number of weeks of study time to reach General Proficiency in Speaking (S3) and Reading (R3).
Full detailed list follows after the map: (the article continues after the ad)
CATEGORY I: 23-24 WEEKS (575-600 HOURS)
CATEGORY II: 30 WEEKS (750 HOURS)
CATEGORY III: 36 WEEKS (900 HOURS)
CATEGORY IV: 44 WEEKS (1100 HOURS)
- Persian (Dari, Farsi, Tajik)
CATEGORY V: 88 WEEKS (2200 HOURS)
- Cantonese (Chinese)
- Mandarin (Chinese)
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Sources: FOREIGN SERVICE INSTITUTE LANGUAGE DIFFICULTY RANKINGS