It takes me 3 months to get the hang of a new job. After that the sky is the limit. How shall I convey this in an interview?

I want to be honest while interviewing....because it does take me3-4 months to get into the swing of things in a new job....after I am fully comfortable in the position, I have proven time and time again that I am able to go up and beyond the call of duty. None the less I cannot lie in an interview and say that I will be able to jump into the position and immediately have a grasp of everything. I have come into situations where I was in a temporary position with possibility of extention and was not taken on because they only gave me2 months to prove myself. I was told on2 occasions that I should have conveyed this weakness in the interview.