Baby food

When did you all start your babies on baby food or cereal? My daughter is 3 and a half months and doesn't seem to be satisfied with just breast milk anymore. She is hungry faster and eats longer to the point where my sitter ran out of milk the other day and my daughter refused the back up formula I pack for her. (It would have been her first time trying it) luckily it was at the end of my shift so she didn't have to wait longer than an hour but it scared me. Obviously I'll be sending more milk but is it too early to start cereal or fruits and veggies to maybe keep her full longer?