I've been learning Dvorak for one month now, switching to Qwerty for conversations with friends and I've noticed only a 5-10 wpm loss from my Qwerty.
I tracked this with Typeracer.
Starting Qwerty speed: 115 wpm (average)
Starting Dvorak speed: 15 wpm (average)
1 Month Later, Qwerty speed: 105-110 wpm (average)
1 Month Later, Dvorak speed: 75 wpm (average)
You really shouldn't give up using Qwerty (I think) because it's useful to retain the skill you've developed over many years time. I never for a second thought I would "forget" how to touch-type Qwerty while learning how to be proficient in Dvorak (and I won't consider myself proficient until I reach my Qwerty speed), and all that actually happened was that I lost a little Qwerty speed (simply from less use), and I expected that to happen when I started with Dvorak.