My mom is in the hospital with anxiety and delirium. She took a fall and hit her head. How long before I need to do something legal?

Legal action because the fall was at her AL? How did the Fall happen?

What kind of legal are you speaking of? Are you her POA? Are all her legal documents set up? Hopefully so, if she is incompetent you will have to pursue guardianship.

Has your mom been dx with dementia?
If so, has she been deem incompetent?
How old is mom?
What is mom's living situation, living alone, living with you or someone else?
What level of care does she need?

We need more information to help you better.