Live in a coercive language environment where only standard putonghua or Mandarin is spoken.
You will need at least 2 years to get up to speed, just to get by with the day to day.
If you want to speak in a professional capacity, that requires learning the written word, and watching plenty of Chinese programming, to hone the understanding of Chinese culture, history and civilization.
Without that understanding, the Chinese language will remain an enigma and condemn the speaker to flat and dull speech, contrived and unnatural to a mainland or taiwanese audience.
Forget about attending courses or buying books such as 250 Chinese Characters/HSK Level x. They will at best get you to lonely planet levels of fluency, enough to get by bargaining at the roadside store, but little else.
Take it from me. I went through 12 years of formal Chinese instruction, and passed all my exams. I can speak tone-perfect Mandarin, and I have the ability to keep up with the majority of Chinese programming.
I probably place at the 99th percentile for Chinese proficiency outside greater China. But I’m nowhere near mainlander fluency through lack of practice. I will struggle in a professional setting today, despite lifelong exposure to the language and culture as an ethnic Chinese.