Around 4 to 6
The recovery process varies from one to another and depends on your body's ability to heal and deal with the stresses involved in the process.
Having double is tough because both knees r involved and so rehab is a bit tougher. Left knee surgery is less (not used to drive), more time with r knee etc..But i would say 4-6 wks is a good range of time to plan..
Best of luck...