When would payment be required? Before or after the tutoring session?
Before is kind of the same as this problem:
People are often shuffled into rigid 1 or 2 hour timeblocks. This wastes money for people who only have 25 minutes worth of questions. It also leaves people with 90 minutes worth of questions high and dry in a 1 hour session when the tutor has to desperately get to the next student in time.
Because it requires students to accurately estimate the amount of time they need if they don't want to waste their money buying tutoring time they end up not needing. However, payment beforehand actually ensues payment.
After: What's to stop the student from not paying? There's not as much accountability online as in real life. A student could take advantage of a tutor by utilising their services, then not following up with payment. This problem aside, obviously payment after the tutoring session is ideal because students can then, as you say, only pay for the time they actually used. Also, if the tutor sucks and you realise this straight away, you only waste a few minutes worth of money and can excuse yourself from the session with a quick excuse like, "Sorry, I have to go feed my cat now."
Edit: I'm making a conscious effort to be coherent right now, but I think I'm losing. If anything doesn't make sense, let me know and I'll fix it later... for now... zzzzzz time.