What are the Medicare standards and procedures for mandatory discharge from a SNF?

My mother has bee at a skilled nursing facility in MA for about 3 weeks, following a hospitalization for a broken rib, collapsed lung, and complications from the foregoing. The facility has been providing good physical therapy to improve her strength, which we would like to continue for as long as possible. The facility social worker is telling us my mother must be discharge to a long term care facility soon. Can you please send me the provisions of the Medicare regulations and manual that list the criteria for discharge in these types of situations, as well as describe the procedures or protocols that are used to make this determination?
Thank you.
Herb O.