I would not relate returning to school to the school calendar - I would relate it to her functioning and overall outlook toward adaptable change. Has she given up partying? Is she practicing reasonable sleep hygiene (going to bed and getting up at times amenable with college)? Is she successful with her job and getting good reviews? Is she working with a therapist and/or psychiatrist toward tangible goals?
Also, is she driving? People with anxiety who avoid driving never do well. Adolescents who avoid driving are not setting themselves up to leave the nest easily. That is a big red flag, and were it me, I would want her driving as a marker of adaptability and as evidence of energy expended working on the anxiety issues - actions speak louder than words.
If she is able to do those things and sustain them for a reasonable period of time - reasonable to you and her - then that would be a good time to go back. But simply throwing a dart at a calendar and having her do some perfunctory tasks in the meantime with no skin in the game is unlikely to be successful.