The Foreign Service Institute (FSI) created a rubric to measure relative language difficulty. The idea behind this rubric is that the more closely related a language is to your mothertongue, the easier it is to learn. For example, Scandinavians have such an easy time learning English (and vice versa) because their native tongues are so similar to English.
That's why FSI classifies languages like Dutch, Swedish, and Norwegian as "Category I". Learners can expect proficiency in Category I languages relatively quickly: after some 600 hours of language study.
While Tibetan doesn't make the list, we can make the educated guess that it falls in the most difficult category: "Category V: Languages which are exceptionally difficult for native English speakers." Why? For one, this is how many of its Asian language peers, like Chinese, Japanese, and Korean are categorized.
For another, the significant difference between "How to speak Tibetan" and "How to read and write Tibetan"—diglossia—makes the language more difficult. And Tibetan's diglossic peer, Arabic, is also categorized as a Category V language (much for this very reason, we can assume).
The FSI estimates that learning a Category V language takes some 2,200 hours of language instruction. This is a serious number. For comparison's sake, a student in the university system can graduate with a mere 280 hours (taught in the English medium, no less). That's well short of FSI's suggested number needed to attain proficiency...
Hours of Tibetan Language Instruction
Check out the full list here: http://www.effectivelanguagelearning.com/language-guide/language-difficulty