Volunteer roles do not require much time (maybe 30mins a month). It is entirely up to you how much time you allocate for your mentors. You can either arrange to hold catch up meetings with your mentors as a group (once a month/quarterly or as often as you like) to see how things are going or you may prefer them to email you to keep you updated or raise an issue.
Essentially your role is to provide a known, friendly face within the programme that students can go to if needed.
All mentors are trained about putting boundaries in place, to know their limits and when to refer. There is also a Peer Mentoring Blackboard where mentors can ask each other questions.
For the paid role you are required to meet with your mentors for 30mins a week or fortnightly depending on how often you require study sessions. This is to provide your mentors with some background and content information as to what you want them to achieve within the study sessions.