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:
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
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
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
Category IV: 88 weeks (2200 hours)
Languages which are exceptionally difficult for native English speakers
Arabic, Chinese (Cantonese and Madarin), Japanese, and Korean
What’s fascinating is that the document includes estimates for conversion courses like Spanish-Portuguese (14-18 weeks) and Malay-Indonesian (10-12 weeks).