Web Designer & Developer: Is it possible or realistic?

Thanks. So, it's definitely possible. what i'm wondering then is about how long would it take for someone who is already trained in design to be proficient in developing/coding a website?

I have the opportunity to learn more about web development but I'm unsure of how long it will be before I can design and build a website well. I'd love to hear what other people have learned through experience.

thanks for any input!