DD1 was in her room at 7 weeks and slept through from the same night. DD2 would have been in her room from birth if MIL was not visiting. She was 11 days old, and is now 5 weeks and sleeping through.
I am an extremely light sleeper, and finely tuned to my children's noises. DH and DD1 have sleep apnoea, and snore like trains, with long gaps... so it is just too noisy for DD2 to get any sleep other than in her room. Co sleeping is recommended partly to help regulate breathing, so I did not want DD2 to copy her daddy or sister!
I have a very good monitor which includes movement, both babies good birth weight and no health concerns. All naps except in pram and car seat are with the monitor on.
I actually prefer to go to the nursery to feed - can get comfy, keeps me awake as I would nod off in bed, everything to hand... nappies, chocolate etc.
Although she is sleeping through, I still respond to any odd noises, coughs etc.
I do not leave her to cry, and seem to wake just before she does, and sleep well myself inbetween times.