Its hard to say since some of it deals with talent, the right type of training, your environment, etc...
^ thats a good post to help you understand what its like. Its different for everyone but a google search of how many hours pros practice will give you a good idea of how much time to spend. Good luck!
There's also the difference between competition surfers and free surfers. Some pros do both, but because of the highly demanding pro circuit travel it can be tough and many pros focus on one area. Competition surfers seek to understand how to optimize their scores in judged competitions while free surfers seek the best waves and work with pro photographers to distribute their work (they tend to make livings off sponsorships, earnings from videos/pics/awards, and their own personal branding).
I realize I'm about 4 years late to your question, but if anyone is looking to become a pro surfer the best thing you can do is ensure you live near good surf (keep in mind not every beach has good surf, even in the best locales). The next thing is ensuring you have a schedule that allows you to take advantage of living near the surf (checking the surf, getting to it, and prepping to surf is a task unto itself). Then you have to actually put the time in, day after day after day. If you're still in school you'll benefit from living somewhere where they have high school surf teams. If you're out of school, or older than 16 or so, and you haven't been surfing for years already at a high level you're probably not going to make it as a pro competitive surfer (just the reality....it's a ultra competitive field with new comers changing the scene and redefining "good surfing" constantly). But, if you're older or out of school AND have a lot of experience surfing and practice all the time and refine your craft incessantly, then you could potentially make it as a free surfer....provided you're willing to risk your health and life to make it.