My understanding is that they're fractionally better speedwise and a bit better for RSI issues, but not enough to justify switching, it would take forever to make up the lost productivity while re-training.
A few weeks to reach a solid level, most likely, provided you actually practice.
If you type a lot of boilerplate code (which really is the only area where typing speed will hurt you) you'll notice a difference, but I'd argue that fact in itself is something you ought to fix.
I taught myself workable svorak (A5) in a weekend on IRC, then polished it over a shorter period after to feel comfortable. The good thing about SvorakA5[0] is that it uses AltGr + regular keys for special characters, which is really nice.
I don't recommend learning dvorak or any variant of it for the typing speed unless you do dictation or whatever, because you're not getting that much from it in any profession that requires you to think anyway. Personally, I just feel like I'm destroying my hands less when typing, which is a good enough reason. They're already some level of broken from having played strategy games too much, so I feel like I should take care of them more.
(Obviously Svorak is the Swedish version of Dvorak, so for other languages you'd have to find something like A5)
Edit: I recommend playing "Typing of the Dead"[1] if you can find a way to do so. Great way for polishing your use of a layout.
It's less than you might think. I did a complete switchover to Dvorak to ameliorate some wrist issues and was back to my normal speed in about 2 weeks. I surpassed my qwerty speed by about 20% according to my own tests before and after.
That said, I'm back on qwerty because my wrist is OK now and it's just less hassle when using multiple machines over which I have less control.
Also, anecdotally, when I made the switch I was in my mid 20's. I tried the experiment again (albeit with Colemak) a couple years ago in my late 40's and after 2 weeks I was still struggling so gave up.
I don't know if age had anything to do with it, but that's one big difference between the 2 times.
I had major wrist issues and made the switch to programmer dvorak around 2 years ago. Took around one to two months before my wrist got better and about the same time before I didn't feel constrained by the new layout.
Now two years later I would have to say I'm at about the same speed I was with qwerty, except for when I have to write numbers with the horrible positioning on the programmer dvorak layout :)
Would never recommend someone to make the switch unless they have problems with RSI.
