The short answer: A LOT of practice. And not just practice! Learning how to make better art, taking art classes, reading about art, making a lot of terrible art, making good art, and just not giving up on drawing/art.
The long answer: Sadly There's no quick and easy answer to the question of "How much practice?"
I've been drawing and making art for maybe about 14 years now. (and even before that I drew stuff, so in a way I've been drawing all my life) I'm both self taught and have been taught by various teachers. (from jr.high art classes to amazingly hard college courses)
I usually "practice" drawing at least every day... but that practice is usually just doodling random things in my sketchbook. (or just looking at pretty pictures and reading stuff) >_>
But that's not how I got good at drawing.
I got good at drawing by learning how to draw. (Even if it was learning how to draw boring stuff)
I found teachers, and classes, and art books, and tutorials. I learned from them and applied what I learned. I learned how to see shapes in objects, and how to draw those shapes. I learned how to pose subjects, and failed at drawing what I saw over and over and over again.
I drew from real life, and from pictures, and from my imagination. I filled sketchbook after sketchbook after sketchbook with my doodles and drawings and failures and successes. (and I eventually started posting the work I was proud of online)
I practice a lot, but I practice what I've learned. (and I'm still learning)
So the question is less: How much practice? But more: How much do I need to learn? (and from a quick glance at your gallery, you have a LOT to learn)
I love this sort of artsy style for a show that's so colorful and cartoony. This has a sort of elegance to it, and it's simple but uses it to an advantage. This is something I would hang in my living room or hallway. Are you going to do a series like this for all the princesses? That would be really cool. Very nice.
Oh gosh, thank you so much. (I'll hopefully have prints of this available for sale later this year)
And as far as doing the rest of the princesses in this style... well... I hadn't even thought about it, until reading your comment... (and seeing how popular this one painting has turned out to be) >_>
Now I think I will be doing a full series of paintings like this for all the princesses. (..as long as I don't run out of paint again...)