There comes a time when mentoring looses its effectiveness and/or appropriateness. To mentor, most generally, is to model behavior. Knowing when to switch up my approach or exit will help me not to outlive my usefulness.
Mentoring is no longer effective when my mentee desires to be someone different than who I am, to go further than I have gone, expresses different core values, wants to be a pioneer/charts his own path, mistrust my authority or mistrust authority in general.
I maintain usefulness when I switch to coaching, connecting my mentee to other relationships that may prove helpful, cheerleading.