To Draeder 1,
First, I think it's terrific that your mother is "doing great" and the professional response above makes sense based on my 97-year-old mil's experience after breaking her hip and having successful surgery to put a pin in. I doubt there's any "rule of thumb."
Her surgeon supplied the rehab prescription for the rehab facility. At a certain point she could have gone home and continued physical therapy there. However, she chose to stay at the rehab facility which had a reputation for outstanding physical therapists who worked with her daily--as opposed to someone who might not be outstanding and wouldn't come daily. All covered by Medicare until she reached the level where it was determined by the social worker and team that she could return home and function basically as she had before the broken hip.
Because you mention hospice, your mother's health situation sounds different from my mil's. She was in very good health. So I don't know how that enters into the equation. I do know the surgeon insisted that she put no weight on the broken hip for 90 days--so that meant 3 months in rehab. plus the extra 6 weeks of rehab once the hip could bear weight.
Being in a rehab facility is not easy, being home always feels better. But my mil is disciplined and decided daily, excellent therapy meant more than going home for the reasons in paragraph 2. Hope this helps. Obviously every individual is different.
(That said, I've detailed my mil's amazing recovery in another answer to: "What is the Treatment for Broken Hips in the Elderly?" here on caring.com or you can get a chronology of her rehab on my blog: http://helpparentsagewell.com.)
Wishing you and your mother the best--