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).
Pacifiers are recommended throughout the first year of life (and after one month of age for breast-fed infants). The pacifier should be used when placing the baby down to sleep, but should not be reinserted once the infant falls asleep.