It depends what you are talking about. You really need to give context. Maybe 'over' is best.

Hi Owais In six months.

In 6 months

The response rate was 60% in the last six months! Which means the evaluation of certain task scored 60% in the past period of time!!!! When you mention "at six months", probably it may be in the near future! Let's say; The response rate should be 60% at six months. Guess this will be helpful for you! I request to have another opinion too so that I can update myself with a different idea! Cheers...

Hi Owais. The correct phrase is "The response rate was 60% in six months"