We did CIO with DS at around 4 months, but it was more like fuss-it-out than full out crying. We had to do it again at around 11months & that involved crying and took about 3-4 of days to settle down. You can give your DD a few minutes to see if she will settle down but if she is crying quite a bit then I wouldn't do it since she is too young. Also make sure she is not hungry or wet or in discomfort before letting her settle down on her own.