SNF liablity to accept residents

This is a question that has come up recently at our SNF. How long is the SNFs liablity to accept a readmit? Say a resident went out 3 months ago, and the hospital wants to send them back. If a bed is open do you have to accept them? I have always heard you have a 30 day liablity if a bed is open and you can meet their needs. Our corp. is saying it is forever, you must accept any resident that has previously been there if a bed is open. Anyone know for sure? Is there a reg that deals with this issue?