There are 3 factors that determine how long will it take for you to learn Thai language,
1. Native language interference (and prior linguistic experience)
a. According to The FSI, US Foreign Service Institute. Which divides languages into 4 groups of difficulty for English speakers.
b. Thai language is in Group 3 and it takes 44 weeks (1100 class hours) for a student to reach “General Professional Proficiency” 
c. Thai is considered a “Hard language” – Language with significant linguistic and/or cultural differences from English.
2. Language learner’s ability
a. There are four important abilities that Language learners can improve to learn a new language, like Thai, without needing special talent. 
b. According to Smart Language Academy, Do You Need Talent to Learn a Foreign Language? article.
c. The more strength you have in these 4 skills, the faster you’re going to be able to communicate in Thai. The four skills include,
c.1 Phonemic Coding (the ability to distinguishing phonemes by using a phonetic script)
c.2 Grammatical sensitivity (the ability to identify the grammar in a sentence)
c.3 Inductive Learning Ability (the ability to generalize patterns from one sentence to another)
c.3.1 Rote-Learning Ability (the ability to memorize lists of words paired with translations)
For example; You might have a strong skill in memorizing the vocabulary but weak in putting them in the structure. So you know to focus on practicing your grammar skill while strengthening your vocabulary skill.
3. Time spent in language exposure.
a. To learn Thai faster, spend more time with the language and culture. 
b. If you only learn in class, you’ll only improve during class time, which is usually just two hours a week. 
c. But if you do things like watching Thai movies, reading Thai on Twitter, listening to Thai music, eating at Thai restaurants, or talking to Thai friends, you’ll get better faster. Plus, it’s more fun!