Been through this with two children and have one son living at home this semester as he completes his intership locally and graduates from college.
It is about respect and good communication. If they haven't learn respect and good communication by now, you have more serious problems than a curfew will ever solve.
I am a mom that doesn't fully sleep until everyone is home. (Anyone relate to that!) My children all know this.
Before they left they would tell me or write a note and let me know what time they thought they would be in and where they were...or at least starting at.
If plans or time changed they called. I lost one child who was killed by a DUI driver, so they know if they drink they stay where they are (friends apartment and call to let me know) I have always offered to come and pick up but that offer has yet to be taken up on! Ha.
No curfew. If they can't communicate and respect you then you need to provide a reasonable deadline to find their own place.
But lighten up, 21...come on, Time flies too fast and she sounds like a good young adult. Don't blow it and cause resentment towards you.