The first question the OP should ask is, do you really understand what a lesson is because a lesson is different from an air exercise?
Your question, and most of the responses, seem to view a lesson as time in the air. A lesson starts when you start the pre flight long briefing study and finishes when you have completed the air exercise and then completed the final self study revision and preferably a test upon that exercise.
Lets say (and this is just a time example) you went to the flying school at 9am and started reading and studying the long briefing for Ex 4 and Ex 5 (your first major air exercise) by 10 am you would be ready for the preflight briefing, for your first Ex 4 briefing I would allow 40 mins so at 11 am you should be airborne and by 12 you would be back at the school and with a 30 minute debrief taking you to lunch. At 1 .30 you would be ready to spend 1 - 3 hours revising the last exercise and preparing for the next exercise or Ex 4 part two, so by 3.30 you would be ready for lesson two and you can already see at this stage that to do the job properly you can only do two lessons per working day up until the circuit.
At the circuit stage though once you have got the basics and are not learning basic theory and are really just learning to land that frequency could be increased to 3 -4 air sessions perhaps, depending on the student, their age and abillity and these two aspects have to be measured by the instructor. You should also consider that you will generally learn more before lunch especially in a hot climate.
The first lesson I try to impart upon FI candiates is that if you want to be a good flying instructor you need to understand that an aircraft is the worst possible classroom and that serious learning takes place on the ground and not in tbe air.
Most poorly trained pilots get involved in tbe rush to complete 45 hours rather than the progression of each air exercise, one at a time (usually) at a pace dependant on their own ability and understanding of the completion standard for each lesson and air exercise.