Are these sentences OK?
You have been saying the same thing for the past 6 months. / You have been saying the same thing since 6 months.
I have been hearing that for the past 6 months. / I have been hearing that since 6 months.

Question

Are these sentences OK?
You have been saying the same thing for the past 6 months. / You have been saying the same thing since 6 months.
 I have been hearing that for the past 6 months. /  I have been hearing that since 6 months.

Answer

The first and third sentences are good. The second and fourth sentences would be ok if you add "ago" to the end: "...since 6 months ago."