If a child is in the top 5% of their class how intense should their 11 plus preparation be?
AB is quite right.
5% is dependent on many factors...
Cohort in class, cohort in year and cohort taking the test. Schools have varying size classes so top 5% of a 3 form intake i.e 90 children in year and a one form entry school i.e 30 in year, would differ. Also they may be in the top 5%, but is that in all subject areas or just one?
If the test they are to sit are VR or NVR and they have never come across these previously, preparation would of course be an advantage.
But do you do it slowly and steadily over 9-12 mths, or cram in 4-6wks?
Varies by family; parents, child and their individual circumstances. I don't think there is a right or wrong way. Whatever works for you and your child, is the best way for you.
Only thing I would advise is by starting earlier, you can check for gaps in learning, especially if tests will be in Maths and/or Eng. If there are any, you have time to bridge them.