We recommend that you and your Mentees agree on a schedule that works best for both of you. The most important thing is to discuss your mutual expectations about time and communication at the start of your relationship.
Keep in mind that mentoring doesn't necessarily require large amounts of your time. Even brief phone calls or e-mail exchanges with you can make a big difference to your Mentees. The annual meeting also offers a convenient venue for getting together with your Mentees over coffee or lunch.