My LO is 4 weeks old and has had a dummy since he was a week old, but isn't overly attached to it. It works for sending him to sleep about 50% of the time, but others (like right now) he sucks it for a bit, spits it out, asks to be fed and falls asleep on my boob.
How long does it take for a dummy to get really established as a means of getting to sleep? I don't know if its worth persevering with or not
A dummy is never going to pacify a baby who wants a feed - regardless of age.
In this newborn stage mostly baby will breastfeed to sleep. It's just the way it is. Offer the dummy regularly if you want to establish it (which I would, dummies are AMAZING) and as you leave the fourth trimester baby should be easier to not to feed to sleep. So 3 or 4 months ish, as a gradual change.