I have a sublet-tee who agreed to a 40 day notice on moving out. She also agreed to stay more than just the summer (I live at the beach in California). She moved in at the beginning of May. Now she is trying to leave with only 25 days notice. What can I hold her to? Does she need to pay for this last month in advance?

Do you have anything in writing and her signature?

Yes, the forty day notice is in writing with her signature.

Then you have her. She will owe you for the entire 40 days. Good Luck