How long will Medicare cover PT in a long term nursing home patient?

It was with a heavy heart that I agreed that my Mom would have to stay in a nursing home after suffering from a severe UTI and dehydration which weakened her terribly. She initially went into the rehab center of the nursing home (she was in AL for 2 years) in an effort to regain strength in her legs so that she could walk (required in her AL). Since she didn't have the required 3 night stay in a hospital before being transferred to the nursing home, she is private pay. I've been trying to find out how long medicare will cover 80% of her PT/OT and no one can give me an answer. I know if she went in from a 3 day hospital stay, Medicare covers 101 days but is it the same for a permanent resident? We are already paying a gazillion dollars for the nursing home and while I'm trying to watch every penny, the nursing home seems to be spending it with no care (different meds, ambulettes).

Thanks for any info!