There is an analogy you can use to help understand more about UCAT as a skill-based exam, rather than content-based. Think of it as learning to ride a bike - some people can instantly find their balance, while some need to fall and fall again to learn how to do it. Similarly, there are people who only did a few hours of prep before the exam who came out with impressive results, while for someone else, achieving a similar feat required many more hours of practice. I agree with Sine that companies who recommend such hours are only doing so for business. This doesn't mean you can't study that much - for some people, that much of practice will still not be enough, and for others it is excessive.
Regarding the title of this thread, there is no set amount of hours that will guarantee anything because it is skill-based. It's like how you can be very good at riding a bike after many years of doing so, but that doesn't mean you'll never fall over again, or be able to navigate any bike trail with ease. There is obviously a correlation between practice and results, but there is no 'guarantee' as such, unfortunately. Some people can easily complete 55 AR questions in 13 minutes, while for others they will require more time to be able to reach that level of skill.