I switched from QWERTY to Colemak about three months ago, and I'm finally back up to my old speed! That's about 110 WPM on TypeRacer.
I spent two weeks practicing Colemak with the TypeFaster lessons before completely switching to Colemak. For a few weeks I spent a lot of time on TypeRacer, maybe an hour a day. Since then I've only been on TypeRacer a few minutes a day, but I type for schoolwork and other things several hours a day.
There were a few points along my progression when I was afraid I had peaked, but so far I haven't reached a serious plateau. I'm hoping to get my average significantly above my old average, maybe up to 130 WPM.
I can say that typing on Colemak seems easier than Qwerty in terms of finger movement. I'm sure that's somewhat due to the placebo effect, but I still think the layout significantly reduces my effort. The motions feel a lot nicer.
For comparison:
Old (QWERTY) TypeRacer
New (Colemak) TypeRacer
So if you're considering switching to Colemak and are worried you won't get back up to your QWERTY speed, don't worry - you can do it!
[Update:] One year into Colemak, my TypeRacer average is up to 125 WPM, and my best speed has increased to 147 WPM from 139 WPM. I can't be certain this change is entirely due to the keyboard switch, but I'm sure at least a significant portion of it is.