For us it was more than what we had originally expected. My dc was really quite good at the start of year 5 but just going through the amount of books we had took an immense amount of time. And you just can't do it all at once - we found my dc couldn't do more than 2 hours in one go (due to his vehement objections but also loss of concentration and motivation and consequently loss in quality of his work). So if we missed some days (or occasionally weeks) we just couldn't catch up on other days. That's why I would advice slowly but surely, space it out, do every day a little rather than leaving more for later, or else you will find that you run out of time. It is much better to start early in year 5 and if you see your dc is good, you can slow down later, but if you do slowly first and find your dc still needs quite a bit of work to get satisfying results, then you might not have enough time left towards the end. If you get all the cgp books, fptp and some other ones, this is more than a year's work. Bear in mind it is not enough to just rush trough the books to have them over and done with. I believe it is effective if you go slowly and explain along the way, so in reality each 10 minute or 50 minute test takes longer. But don't be deterred by the amount of work. I believe majority of kids study really hard the year prior to the exams even if they (or their parents) do not admit it, putting their pass down to their offspring's natural inborn genius
of course there will be kids who will fail despite studying hard, but I guess less so the other way round (those who will pass despite not preparing hard).