Hey Carollynne,
Great question! I remember asking this myself. I think the answer is different for everybody. I share WaveRider's mindset that hard numbers are not a good answer to this question. I know that that kind of stinks because it isn't a straight answer, but it is also cool because, like with everything in the Forex, you are your own boss, you move at your own pace, etc.
I think you should trade in a demo account until you reach consistency where you're winning more trades than you're losing on a regular basis. Like Pharoah said, it's about being comfortable with the returns. Not only does this give you confidence in your ability as a trade, it shows that you are using strategies that work, that you know how to use effectively, and that your equity management technique is going to keep you safe. Now, everyone judges consistency differently, but the point is that you want to make sure that you aren't just getting lucky with your trades then moving forward with stuff that might not really work. Each person is different in this aspect. I've seen some people jump from demo to live in two weeks, others take their time and move over after a few months. It personally took me two months to get the confidence to move to a live account. My main focus was being able to predict ROI on my investment and being comfortable with my ability to execute the strategies properly before I wanted to make the big move and I think that taking my time here made all of the difference.