We did what I call a 'modified' CIO with DS.
We would put him down and if he started crying, we would wait five minutes to go back in. Gave him a quick kiss and reassured him everything was fine then left.
If he continued to cry, we would make it ten minutes, kiss, reassure and leave.
The next time, it was 15 min.
I don't remember how long it took us, but I remember he was probably 3-4 mos old when he started sleeping through the night. I'm not sure if this had to do with the fact that we did a modified Babywise or modified CIO or both. Probably both though.
This is what we did with our DD. I started it at 4 weeks old. Some people think that's too young, but it absolutely worked with us. DD slept through the night by 8 weeks old.
Before she was sleeping through the night, when she woke up crying and it was meal time, I would nurse her (of course). But if she was well-fed and dry and warm, I'd let her fuss.
It took about a week - first she'd fuss for 30 minutes (with some pats from us), then 20, 15, 10, etc.
I agree with the PP who said there's a big difference between hard-core CIO and fuss-it-out. If my DD ever screamed bloody murder, like she was absolutely terrifed, I'd go in. But if she did the kind of rhythmic, ehhh-ehhh-ehhh, tired cry, I'd let her fuss. I think that's what most people mean when they say "CIO" anyway.
She is such a wonderful sleeper now!! It was soooo worth it. She goes down awake, cuddles up with her Elmo, smiles at us, and goes to sleep. She loves her crib and asks for a "Nooze" (snooze) when she gets tired. She is the happiest little girl. I understand when someone says they don't want to do CIO, but I HATE when I hear people say that CIO kids are maladjusted, hate their cribs, feel worthless, etc. (Not that you're saying that

- I have just heard it from others).
One of the nicest things about CIO (or modified CIO) is, once they can go down without crying, you know that something's *really* wrong when they do cry. Every few weeks my DD will wake up and cry and I go right in to her room. Why? Because I know something's wrong - like teething, earache, wet diaper, etc.
Don't regret it at all.