Work from the start, until the end.
Complete as many questions as possible. Obviously, you will need to do more for Core 3/4 as opposed to the pathetically easy Core 1.
Maths has taken up a considerable amount of my time, but I don't tend to think of it in that way. I set myself a task; complete 'x' chapter, complete 'y' past-paper or complete 'z' exercise. The time period varies; if I understand the material, I can breeze through the material (I was doing C1 papers in 15-20 minutes at one time), but if I feel that I'm not confident on one little thing, then I just keep working at it - this can potentially take hours.
In addition, I do Further Maths, hence I spend much longer on mathematics.