We did it when she turned two - cold turkey, no story about giving them to the sick babies in hospital or the fairies taking them (no offence if that's your bag, it just isn't mine ITMS). We just said 'no more dummies, you're a big girl now'. It was tough for a few days, and even months later she would still ask if she was really tired, hurt or upset. Best time to do it - when you're ready for a couple of rough days and feeling strong enough to hold out. And I agree with hj1981 that cold turkey is the best way.
Good luck!