Not that I'm any expert, but wouldn't it take, like, years and years? And I imagine it would depend at least partly on your own bodily attributes, like sense of balance and whatnot.
It really depends on the person. My brotehr picked it up right away, my sister still sucks after eight years so I think it is the person and how they take to it.
I think it is not the number of years but rather the number of days you ski or board in a row. For example if you go for a week every year the first few days you spend getting the feel back then you leave when you are starting to get dialed in. I was able to get like 45 days in a row my first season and it made a big diff. I am no pro but like to think had I started earlier I could have been. Even taking to trips in the same year will really help you get better. Also skiing the same mountain while learning will really help. Once you know where you are going you can focus on the skills.
It all depends on how many times you practice and how much time you put into your skill. I mean everyone learns differently so I don't think there is a definate answer to that question.
It all depends whether you can take those falls. At times, it really got to me and made me want to give up skiing but I keep telling myself I can do it. Now, I just can ski! A professional? Me? I will never be one in my life. It would take ages even if I practices daily. I think I just don't have the skills in skiing.