Most of my private tuition work is one-to-one tuition. I recommend this form of tuition above group tuition as it means that the student has the full focus of the tutor and a personalised programme of study can be devised to meet the individual needs of the student. Group tuition has its advantages too though. For a start it is cheaper, and secondly, the discussion that can only come from a group of peers can sometimes help individual students to understand a concept in their own terms, especially if the discussion is skillfully guided by the tutor. Because of this I offer group tuition too and it helps a great deal if the students already know each other beforehand. I limit the size of groups to four as too large a group can negate the potential benefits.
My lessons usually run for one hour each, although you may ask for longer or shorter lessons if you wish. I find that anything less than an hour often isn't enough to cover a topic thoroughly and more than an hour can be too long for a student to continue to focus when being taught so intensively.
The frequency of lessons is also important. If you are using private tuition as a supplement to your normal College or School lessons, then one hour a week should be sufficient. If you want to intensively prepare for an upcoming exam then you may want more than this, and it will depend on how much time you have until the exam and how much you feel you need to go over. Alternatively, you may wish to study a full A-Level programme for which I would recommend at least 3 hours per week.