The 11+ system is unfair on so many levels. The "system" was never designed to force children to work more during their holidays.
But it exists, so you either face it or ignore it.
Ds sat both the Kent test and the Sutton "superselectives" last year.
For Kent he had to sit GL VR, NVR and Maths papers. Just over 9,000 children sat the test and of those 1,555 scored the "maximum" standardised score 141 in VR which equates to a raw score of approx 80%+. Almost 6,000 children scored over 118, the passmark. By way of contrast only 225 children scored 141 in maths, and 3,660 "passed" maths. The difference is down to how easy it is to prepare for VR - almost half the questions can be answered if you know some standard techniques, the remainder rely on vocabulary which seems to mainly come from a fixed bank of words. So with a few hours of practice it is reasonably easy to improve your score to around 80%. NVR was similar, but not so pronounced - some children really struggle spatially regardless of the amount of practice. For most children the chance of a Kent grammar place rested solely on their maths (and then how close they lived to the school, or whether there were other criteria).
The Sutton selectives ran an initial round of multiple choice maths and English papers which cut the number sitting from 2,200 to 800, and then each school set their own standard form maths papers and a creative writing paper. No VR or NVR in sight. Children had to pass the second round to have a chance of a place, and the places were allocated purely on their ranked score. There are often up to 30 or more children on the same score around the cutoff, so every single mark makes a difference, and dropping 3 marks across 4 papers could mean the difference between a place or being 70th on the waiting list for your preferred school.
The amount and type of prep for each of these exams was quite different. Once you'd cracked the technique for codes in VR, then you can approach the question with confidence, and just a little bit of practice is needed to keep you up to speed over the summer. But for creative writing, it is always possible to continue to extend your style, and there will be some children writing at level 7 at this stage. And there is a difference between being able to write well, and being able to answer set tasks in a limited time.
So in terms of what is needed, it very much depends on what exams you are facing. For Kent we knew ds didn't need to score above 80% in a paper - whether you scored 80 or 100, you'd still get the 141 maximum standardised score. For Sutton, every mark counted. At this stage (with 2 weeks of holiday left) 2 or 3 hours a day would have been total overkill in Kent, but felt about right for Sutton. That said, I do agree that a child should try a couple of mock test days, as managing the full quota of tests on a single day can be something of a marathon. Ds's Kent test involved him being in the test centre for almost 6 hours.
Ds doesn't regret any of his preparation, and apart for some of the tedious VR/NVR which thankfully was only a small part of the effort, he has noticed the benefit across the rest of the school year. The most significant benefit for him has been to understand that if he puts in effort, it makes a difference to the outcome. None of us are looking back at a blighted summer, much less a blighted childhood. There is still plenty of time to have fun in the other 8 or 9 hours a day. That said I do feel for parents who are both working fulltime (or obv single parents working fulltime). This is a very difficult time for them, and it must indeed feel very relentless. Within my circle of friends and acquaintances it was mostly children whose parents were in this category who opted out of the tests.