Quoting Shelby Chappell:" My husband says that 30 minutes leaving our 4/12 month old is ok enough to teach him boundires. I say ... [snip!] ... I dont agree with his methods at all. I hope this shows him that leaving an infant to cry for 30 minutes straight is NOT OK"
you SHOULD NOT do CIO until 6 months at the very least if ever, dont let him care for the baby if he just lets your LO cry for that long, that poor child.
i wouldnt personally ever let my lo cio though, he was a difficult newborn up until 5 months and i wouldnt dream of that. he now goes in his cot with a bottle and puts himself to sleep he learnt to by himself no need for cio.