How long should one tutoring session be?
Some students just have the ability to sit longer than others, and forcing yourself to study longer than you should could cause more damage than good.
When trying to determine how long a tutoring session should be, take into account your attention span and whether you enjoy the subject you need help with. If your attention span is short and you hate the subject, it is not wise to overload yourself with more than two hours per session.
If you are unsure what is best for you, try out a few different arrangements and settle in to the one that seems to work best. For example, you can try a couple weeks of one-hour sessions three times a week; then, follow up with one two-hour session per week.