When can a title ix case evolve into something criminal?

At any time, a complainant in a Title IX case can decide to report the conduct in question to the police.  In fact, under the law, schools are required to advise complainants that they have the right to contact the police to file a report.  That being said, just because a report is made to the police does necessarily guarantee that criminal charges will be filed.  Police will conduct an investigation and then a prosecutor will review the case to determine whether prosecution is appropriate.