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).
Using a pacifier in the early weeks can also cause problems. If you want your baby to take a pacifier, I would introduce it by the time he is one month old, but preferably not during the first two weeks of nursing. Some babies refuse to ever take a pacifier.