This is dependent on the type of program, the objective, and the program managers in charge.
However, best practice for mentoring relationships is 6 sessions:
2 sessions for Familiarization
2 sessions on Challenges & Goals
2 sessions for Future Planning & Reflecting
The best way to know how long your relationship should last is by speaking with your mentee / mentor.
Remember, don’t be afraid to conclude a relationship once it has naturally ended. That way, you free yourself up to match with someone else.