I started at 3 1/2 - 4 months with 5 tablespoons of cereal mixed with 6 ounces of formula twice a day (breakfast and dinner) and now at almost 6 months we are giving her baby food 2-3 times a day, a full jar for breakfast lunch and dinner, and offer her a bottle afterward if she is still hungry, and give her bottles throughout the day in between feedings.
My doctor told me not to until he was 6 months, so I waited till then. Once a day. However, he didn't much like it, so we went on to other babyfood within a few days! After he got better at eating babyfood ( I guess within about 2 weeks) we started feeding him babyfood twice a day.