Of course your DC (and you!) needs to put a fair amount of work, but ultimately its not the quantity, its the quality. Look at the recent thread posted by newbie- I completely agree that instead of hours of papers, which will bore them rigid, focus on your child's weaknesses.
I believe you mean this thread? viewtopic.php?f=18&t=24020
I think although quantity is also required, the best is to pursue quality - you will get some level of quantity anyway... (I do not think you can avoid getting DC to do a wide variety of school past tests, question types, etc). You will see on the various threads that there are various areas that each person found worth focusing on. These become clear depending on the specific exams DC will sit and whether you get much information about those and if you manage to diagnose early on what the focus areas are.
11nuts, what year is your DC? The Sats results and any VR assessment that DC has done will give you an indication of where to aim. Other things to consider are whether many children in the same school will apply and how to deal with the social pressure that you may have before and after the exams, the results, the actual choice of school, etc.
This year I believe that the game is also changing since it's likely that all regions will need to allow children to sit exams and have them marked before you rank the schools on an application form- this will bring the dates earlier but make it less stressful for many people.