How many hours a day studying for gcse or nat5 prelim exams?

[Teeh]

Or mocks, whatever they called. How many hours a day did you ds /dd typically do a month before exams?

Do you mean a month before the real things or mocks? Very different here!
A month before mocks probably nothing other than completing homework. Before real thing more like 1.5 hours a night.

DS is currently spending 1-2 hours a day preparing for his pre-Xmas mocks.

He is then planning on increasing this as the academic year goes on.

This is his choice.

Mocks. We are in October holidays so I am not talking about after school, but days when he has the whole day free