I put a calculation into DS's revision timetable yesterday to work out how many possible hours of revision there are left before he has to sit each paper: 35 hours until History on Monday morning, 87 until Physics paper 2 on 15th.
I suspect that most of your DC will be averaging something similar.
It sounds like a lot, but not when you think that past papers typically take 90 mins each, no including marking. Also, even 10 minutes here or there to learn some sub-topic, soon adds up to several hours. If DS does everything that he currently has planned, he is going to be busy for 80% of those last hours.
The assumptions were that the maximum possible hours to work in a day = 7, reducing to between 0 and 4 when there are exams.