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).
My children have their pacifiers until around age three. At one year we limit it to naps and betime. The only time they walk around with it is when they are out and about and super tired or not feeling well and laying around the house. My children do not have it in their mouth 24/7. I let them get rid of it in their own time.