Hi, what type of visa should I apply for if I want to stay for a duration of 6 to 12 months?

Vanessa, since you didn't get married with your BF, I think you can only apply for L type to stay with him. According to what I know, there is no L type that allows the applicant to stay for 6 to 12 months. The duration of stay for L type (even for a multiple one) is usually 30 days.

Another option is that you find a job to work in the city where your BF is located at. Then you can acquire a long-term resident permit.