totally depends on baby. most say 4 months but a lot of people wait until 6 months. I started at 4 months with fruit mixed with cereal and by 7.5 months my son was eating strictly table food and stuffed blended in our baby bullet. he didn't eat purees for too long.
you normally want to start @ 6mo. 'food before 1 is for fun!' you can add some rice to formula if baby is really hungry. but I'm not familiar with that. I ebf my son, he wasn't interested in food until 11mo. and he's still bf and eats plenty of food. so I wouldn't worry about it.