How Long Did It Take to Be Cleared?

I'm going to have knees surgery in the first half of April of 2013, I'm going to have a full replacement of my left knee. How long does it take to get cleared by your knee surgeon to work out at a gym? I know each state, city, hospital & doctor/surgeon is going to very. Also I guess on how well the surgery went and how well you heal and how well you do with physical therapy. I know at some point once I recover from left knee surgery and go through with physical therapy and at some point I'll have another surgery and that'll be on my right knee which will be a full replacement. I'll have two knee full replacements this year. I'm trying to do some homework here on a time frame on being cleared for exercise.

Thank you in advance.