AW2011, here is my timeline...Hope this helps!
We received your application to sponsor ? on December 20, 2010.
We started processing your application on January 19, 2011.
We returned your application with a letter on January 19, 2011. You may re-apply or resubmit your application when the information requested is provided. Please consider delays in mail delivery before contacting us.
We received the application that you resubmitted on February 25, 2011.
We sent you a letter on March 7, 2011 about the decision on your application. Please consider delays in mail delivery before contacting us.
We received ? application for permanent residence on March 17, 2011 (in Buffalo, NY).
Medical results have been received.
Your interview is December 7, 2011.