Everything in maths relies on each other and to learn the more complex concepts in a topic, a deep understanding of the basics of that topic is necessary.
That's why a one or two hour long session is not effective, because the tutor has to go through more complex concepts before the student has had time to fully understand the less complex concepts.
Our students receive one to two hours of tutoring in blocks of 30 mins, where they learn a topic, do questions with their tutor, and then be given questions to do on their own. Then after they've had time to consolidate their knowledge, they have their next 30 min lesson, and so on.
If our students get stuck during their independent study they also have a mini lesson and ask their tutor questions.