When DS was revising for his mocks last month, he worked in one hour blocks. Maximum 3 hours on a school day (usually 1 or 2) and maximum 5 hours on Saturdays and on Sundays ( more often 3 or 4).
I don't think he found it difficult to build up the stamina to do an hour, and as some of the exam papers are around 2 hours long, being able to concentrate for extended periods is necessary.
From my own school / university / professional exams days, I think the best way is to learn the material one chapter, or topic, at a time and then use the past papers to highlight areas of weakness and to learn how to time yourself so you don't t run out of time in the exam.
The problem with past papers is that there are never enough of them, as the style of questions and the syllabus changes slightly every few years. So, even if these exams weren't all new anyway, doing lots of past papers now would mean that in April and May, there won't be any fresh past papers to work from. And that's when you really need them to start checking what you know, timing yourself and generally honing exam technique.