Ds (EBF) had a dummy from early on - prbably as early as he was being given EBM ('cos he wasn't gaining weight), which was 2 weeks, but I can't quite remember now. He was a real comfort feeder though - I used to be able to unlatch him without "breaking the seal" 
We didn't give him it to go to sleep though - but then, he was always --too- a good sleeper.
I weaned him off the dummy at 6 motnhs as I ready somewhere that it was 6 months or 2 years.