How Long Is A Short Term Stay At A Nursing Home?

How a "short stay" is defined depends upon the reason for her admission to the nursing home. Upon admission to a nursing home, the "resident" and "sponsor" (family) should be given information about the resident's rights, the discharge planning process, and the next steps the nursing home will take to plan for your aunt's return home.  

Ask the nursing home for a "care plan meeting" - you can get a useful fact sheet about this process at www.nccnhr.org or start at their home page and look for "fact sheet"  then "assessment and care planning" at  www.nccnhr.or .
Locate free help from your local Long-Term Care Ombudsman www.aoa.go  (click on Elder Care Locator) and contact your local LTCO program to ask for information/advice on what next steps you/your aunt can take to plan for her to leave the nursing home.  

Often a nursing home will suddenly be ready for a resident to leave when the 20-day maximum stay at full coverage (Medicare) is approaching – how long a resident actually needs to stay will depend upon her care needs, not on this 20-day window.  If they say she needs to leave before she feels ready, ask for the nursing home to put this in writing – a resident also has the right to appeal the nursing home's decision, and these appeal rights should be included in their written notice to your aunt (the resident).
See www.medicare.go – specifically:
www.medicare.gov for more information on patient/resident rights under Medicare.  
I hope this is helpful!