How accurate is the map locator for lost iPhones?

It depends on how much information is available at the phone's location. If the phone is using only cellular, the accuracy can vary from approximate to close. If the phone is using both cellular, WiFi etc, the location might be very close.

We assume that you have enabled Find My on the phone. The phone will need to be "on" and connected to the Internet in order to display any information about its current location.