I've used Colemak for 18 months now, and thought I'd chip in with a data point.

My initial QWERTY speed was roughly 80 wpm in an unsustainable fashion (doing the typing test would leave my arms wearied). I could sustain around 50-60 wpm.

These are my (rough) Colemak speeds at various points in time. All wpm are from a typing test and are unsustainable.

@3 months: 40 wpm
@6 months: 60 wpm (from this point Colemak was no longer an effort)
@9 months: 70 wpm
@12 months: 75 wpm
@15 months: 80 wpm
@18 months: 90 wpm

My sustained speed today is around 70 wpm, and my record hi-games speed is 93 wpm.