duration of tmua preparation

hi all
i have started 11th grade a month ago, and i want to apply for bsc economics and statistics in 2027 (when i enter 12th), so that i can start college in 2028, (after 12th grade)

please help me with the following questions!:

- when should i start preparing for tmua?
- how long would it take?
- is it generally a difficult exam? (im aware this is subjective, but a general answer is appreciated!)

thank you in advance guys :)