I started learning Colemak on Tuesday, May 1.
By Saturday, May 5, I completed all 12 colemak lessons on ktouch set to 96%/130cpm.
At this point in time, I was able to type at about 20 wpm.
Since then, my typing speed has been increasing significantly every day.
5/6 - approx. 25wpm
5/8 - approx. 37wpm
5/9 - approx. 51wpm(!)
* tested using http://labs.jphantom.com/wpm/ and http://www.freetypinggame.net/free-typing-test.asp
I attribute this to spending a lot of time at a keyboard every day, and playing words for practice.
And now for some background.
I stumbled upon Colemak a few weeks ago while trying out dvorak - which was a not-so-pleasant experience. I decided to give colemak a trial last week due in part to the suppose ease-of-acquisition of the layout, and in part to the rather fortuitious timing of Golden Week (a week long block of public holidays in Japan).
Although using a JIS keyboard posed a few early problems, I managed to set up colemak fairly easily on both Mac OS X and Linux. I have also been typing Japanese using colemak for Japanese input, but this hasn't been too good due to the poor placement of K and J. I will probably switch to one of the many alternative romaji based keyboard layouts after colemak gets comfortable.
# I don't plan on using kana input, as the JIS kana layout is actually also rather poor. I also don't like how kana input layouts (including the tens of alternate layouts) end up either using 4 rows, or overuse the shift key.