hi, i did CIO with my elder son at 10mths and am now in the process of CIO with my 6mth old son. i really hate listening to the crying but my 6mth old now sleeps for 12-13hrs and wakes up really really happy so i know he is not being hurt in any way.
Clowe, I always trust my gut. I sit by his door and listen to the crying. A tired cry is very different from an I pooped cry or pain cry (which i more high pitched). If my gut says something is wrong, I will go in. When you say gas pains, do you mean he has a burp stuck? or lower gas pain (how do you know?). The crying itself is hard to listen to - i literally can't do anything (eat, talk, pee) until I know he's sleeping and the crying has stopped. but there are some nights when the cry doesn't sound right and in those cases i will go in right away. you will need a few nights to figure out what cry means what but trust that you will figure it out.
with my elder son, i tried a milder approach where i was in the room with him, but laying down on a mattress and laying him back down every time he stood up but after 2 days and 3hrs of it (starting at 7pm and ending at 10pm) i realized that i was more of a distraction than help. On the third night i decided to leave the room and he fell asleep on his own after 30mins of crying. the following nights, he gradually went down to 10mins of crying (Gina Ford calls it Crying Down - neither of my kids are GF babies but I did read every single sleep solution book out there and tried Baby Whisperer's shush-pat but it didn't work for us), mind you, he Cried Down almost every night for 1mth - a few minutes every night and never over 10mins but there was always crying. After a month, one night when I put him in the crib, he just rolled over and went to sleep and that was the end of the crying (what a relief!!). He is now 22mths and bedtimes are my favourite time of the day. We snuggle and sing a song, I lay him in his bed and he says 'goodnight' and i leave the room.
With #2, the past week has been tough since i am adjusting his bedtime from 9pm to ideally 730 (we are at 830 now). i find if he's over-tired he will cry much longer than if we have a calm day and he's had good naps throughout the day. We had 2 nights where we lay him down awake and he babbled for a bit and then fell asleep without crying but these past few nights we are back to crying on and off for 45mins. We don't go back into the room after we put him down since for us, going in will start the clock back and he will cry an additional 15mins if we go in.
I was one of those people who said i would NEVER CIO...with my elder son, I was alone and pregnant, vomiting while shushing and patting him for up to 2hrs some nights and I couldn't do it anymore. With my 2nd, I have more understanding that it's OK to pick the method that works best for your family/situation. Good luck and hopefully quiet nights are just around the corner!!!