1. 난 여기에 그렇게 오래 있지 않을 거야.

I’m not gonna be here so long. --------> I’m not gonna be here for so long.​

=> Corrected : The sentence is corrected grammatically.

2. 난 충분히 마셨어.

I drunk enough. --------> I was drunk enough​ / ​I drank it enough.

=> Corrected : The sentence is corrected grammatically.

3. 난 꽤 지루함을 느껴.

I feel quite bored. ✔​