Utility is an economic term for a person's overall well-being and satisfaction. Consumers play a role in the economy by maximizing utility, which means allocating their scarce resources, such as limited time or money, most efficiently. Utility maximization is different for every person.
You can start using it right away but it was suggested to us by our midwives to hold off until 6 weeks as pacifier use can mess with their breastfeeding latch (it's a different kind of sucking).
Only one of my kids has been a big pacifier user. He is now 21 months. I wanted him to stop by 18 months, but due to his baby brother being almost exactly 18 months younger, he got an extra couple of weeks. He had already been reduced to naptime, bedtime, and in the car by the time weaning time rolled around.