I type at 130 WPM with QWERTY. Two questions:
1. For those who have done it, how long does it take to learn a new keyboard layout (e.g. DVORAK, others) and exceed your previous speed.
2. Is switching back and forth easy to do (like speaking two languages fluently)?
The hardest part of learning a keyboard layout is the first week. During this week you cannot type on the autopilot at all (even my qwerty skills were fucked up).
I recommend starting learning a new layout during a holiday because it will break your productivity. For training, you can use http://klavaro.sourceforge.net/en/index.html, which has support for Colemak.
After 4 weeks of typing more than one hour per day, I managed to hit the 50 WPM at Colemak. It is also important to type 30 minutes per day using qwerty during this 4 weeks so that you will not lose your qwerty skills.