I thinmk that we did CIO around 5 months or so, when my sons would wake up for an early morning feeding (4 a.m.) that was habit, not needed. For bot kids, the time they actually cried was relatively short. We actually have been blessed with great sleepers: I have no idea as to whether it is nature or nurture, and I realize that I may be eating these words when #3 comes along!
For us, though, the paci was "the thing." My elder son would go to sleep with it and then constantly lose it and start crying. We would be going into our room (one bedroom apt for the first year of his life) several times a night to put the plug back in, and, frankly, it just got annoying. (Hey, we were both students and needed to study!) I heard a friend relate her tale of weaning her 2.5 y/o from his paci, and I was terrified by the fact that he was reasoning with her! E.g. "Honey, pacis are for babies, like your little sister," and he would come back with, "But I'm the baby!", in tears. I decided that that was one future situation I could kill NOW, with my son at 6 months, so, on New Year's Eve, while my husband and I watched "Rocky," our son cried for 3 hours straight before finally putting himself to sleep. It was horrible. But, I had the image in my head of what I wanted to avoid, and that, I felt was more horrible. Night two, he cried for 40 minutes; night three, something like 10; ever after, he was fine. I'm with the commenters above who said, "Whatever works for your family." My husband and I needed the time after our son went to bed and wanted to avoid a horrible paci-weaning experience later. To others it might not matter if they have to re-paci their child, or co-sleep, or nurse, or survive on little sleep. We figured out what was important to us, did what was best for us and our kid, and that was it. My sympathies are with the families who do have sleeping problems, and I wish you all the best of luck!