I've posted in the sec ed section about how many hours is spent on GCSE's each week at school.
For example, 3 hours on English, 4 on Maths etc per week and then maybe 1-2 hours homework.
If I wanted to follow a similar pattern of a two year GCSE, how much could I cut it down based on 1-1 tuition/independent study? Could my son learn the same volume with 2 hours of math rather than 4 hours, or even less? Assume he is an average learner.