I think it depends on the child. My sister has a 6 year old and his little brother will be 3 in April, so they are 3 1/2 to 4 years apart and my oldest nephew was very jealous of his baby brother. He was the baby for a long time (he also has for other older siblings from his father's previous relationship) and it took him about a 6 months to a year to the helpful big brother he is now.