You are going to get two types of responses - some will say "hours of practice" and some will say "you are born with the ability"
I believe the true legendary players have a combination of both.
I'd say the legends of billiards and snooker were born with the innate and natural ability of athletics, precision, balance, hand-eye co-ordination, and most importantly, overall motor skills. If you are not one who is naturally graced with exceptional ability in these areas, there are lots of billiard practice drills and routines to help you develop each one.
Next, the legends who are born with these abilities go from "very good" (which they are without even trying) to "great" by practicing regularly.
There are a hundred ways to look at it, but I think one must consider their "bottleneck" - that is, what is the single biggest skill flaw that affects your game, or tournament performance RIGHT NOW.
That is the one thing you should work on. Once you become proficient in that one area, start the assessment over again, and pick your new biggest bottleneck to performing well.
When I was a kid, I was a competitive skateboarder, competing as an amateur at the national level. I was not born with any ability. I had to practice 3 hours for every 1 hour of practice of some of the "naturals".
After all these years and looking back, I can see that the one thing that most affected my skateboarding abilities, was my physical health and the lack of strength in my legs.
I was relatively out of shape, and my legs were not as strong as they could have been. If i had just spent a year in the gym working on my leg strength, it would have been a game changer for me.
Sometimes its the things you wouldn't think of. (like working out in a gym, and eating health would be the last think on the mind of a skateboarder trying to improve.) I think the same is true for Billiards.