The US Foreign Service Institute (FSI) created a list estimating the length of time required to learn each of the languages taught by the FSI. The estimates are based on native English speakers with no prior knowledge of the language.

Here is an overview:

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Category I: 24-30 weeks (600-750 hours)

(“World Languages”) Languages closely cognate with English

Danish, Dutch, French, Italian, Norwegian, Portuguese, Romanian, Spanish, and Swedish

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Category II: 36 weeks (900 hours)

Languages that take a little longer to master than Category I languages.

German, Haitian Creole, Indonesian, Malay, and Swahili

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Category III: 44 weeks (1100 hours)

Languages with significant linguistic and/or cultural differences from English

Albanian, Bengali, Burmese, Czech, Finnish, Greek, Hebrew, Hindi, Hungarian, Icelandic, Khmer, Lao, Macedonian, Mongolian, Persian (Dari, Farsi, Tajik), Polish, Russian, Serbian, Slovenian, Tagalog, Thai, Turkish, Ukrainian, and Vietnamese

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Category IV: 88 weeks (2200 hours)

Languages which are exceptionally difficult for native English speakers

Arabic, Chinese (Cantonese and Madarin), Japanese, and Korean

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What’s fascinating is that the document includes estimates for conversion courses like Spanish-Portuguese (14-18 weeks) and Malay-Indonesian (10-12 weeks).