baby cereal

I started my baby on cereal today. She's 4 months. I know some doctors say to wait until 6 but mine said it was fine and my baby really seemed to enjoy it. But I'm wondering how many times a day I give it to her? I forgot to ask her doctor this. And what times? She still drinks about 3 ounces of formula/breast milk every two hours.  Ftm and I really have no idea what I'm doing. 🙈