A few years ago I switched to dvorak. It was mostly because I wanted to try something new. I never made an effort to write qwerty / touch efficiently, so I wrote rather fast, but "wrong" (using only 1-2 fingers on each hand).
When I switched it took me about 3 weeks or so before I felt rather comfortable using dvorak. I still couldn't write nearly as fast as I used to write with qwerty, but it was at least improving. I'll say it took me 2-3 months before I could write naturally in dvorak. As others have commented, it totally screwed my qwerty-skills. I had just started in a new job, and the others working there probably weren't impressed by me not being able to write on a keyboard
However; I spend a lot of energy writing dvorak correctly, using the right finger at a given key. Later, when I got used to qwerty again, I found this had helped me write qwerty more "correct" also!
I now can write fast using both keyboard layouts without any problems. It's like switching your brain over to either of them. Personally I haven't used dvorak for a while, but I can still imaginge the layout in front of me when typing. I really prefer dvorak, but it's a bit too much work changing short keys in programs, etc.. So I'm sticking to qwerty for this reason.