How long does it take to learn ASL?
Learning ASL is not easier than learning spoken French or any other spoken language. It takes at least six 3-credit ASL courses over the span of 2-3 years to attain a beginning-intermediate skill.
To attain an intermediate-fluent skill, it takes another 2 years in the ASL/English interpretation training. To become fluent, it takes some time, at least a few years or more, after the graduation from the ASL/English Interpretation program.
Below is an example of the curriculum of a "ASL and Deaf Studies" program outlining credit-based courses at a post-secondary institution.
ASL and Deaf Studies program
Based on the Signing Naturally Level 1 curriculum, the level 100 consists of two semester courses: ASL 101 and ASL 102. Each 3-credit course consists of 60-65 credit hours (120-130 hours total for two semesters).
The level 200, based on the Signing Naturally Level 2 curriculum, consists of two semester courses: ASL 201 and 202, each 60-65 credit hours (120-130 hours total).
The level 300 consists of two 65 credit hours (that is, the total of 120+ hours).
In some programs, an additional courses may include such classes as two 45 credit hours of the courses in the area of Deaf culture and studies, "Becoming an Ally", and so on.
Completing these courses with all passes grant you a diploma in "ASL and Deaf studies". Or, a BA degree with a major in ASL and Deaf Studies. Horray! Congrats! But, you are still a signer, not qualified to be an interpreter at all.
ASL/English Interpretation program
To pursue further studies to become an interpreter, it takes 2 more years of full-time in an ASL/English Interpretation program at a post-secondary institution to receive a diploma or equivalent.
And, those interpreters never cease to learn something new (grammatical structure, vocabulary, etc) for the next many years in their interpreting career.
Also see how to pick the right program for sign language class.