When most people fly into any city that is not their home, they rely on taxi services to help them get around the new city. There are of course other options, such as public transportation, but most people choose to go with a taxi instead. With a taxi, there is no waiting for scheduled routes; you can get your bags put in the trunk instead of having to juggle them on a subway platform. You also have the comfort of knowing that you don't have to navigate your own way around a strange city and can simply ask your driver to take you to your destination.
When you're coming into this new city though, you have be sure that you can actually get a cab so that you can get where you're going. So, if you fly into a new city, and need a car to come and pick you up and take you to your hotel, you need to know how soon you should call them to book your ride.
One thing that you can use to help you determine when you should call the cab is whether you've flown into a city on an international or a domestic flight. When you fly in on an international flight, the process of actually getting through customs and getting your bags is going to take much longer. This means that you need to allow more time before you bother calling your cab. If you call them too soon, they won't wait forever for you and you'll just end up having to call another car.
Whether or not you even have to call a cab actually depends on what airport you're flying to. For instance, at Pearson International Airport in Toronto, there are always standing cabs waiting for any passenger who wants to take them immediately outside each terminal. Finding that out about your destination airport can be an important piece of information.
Another option is to call for your cab before you even board your flight. Tell them your plane's arrival time. Cab companies are used to picking up people from the airport. They should be able to make a reasonable assumption about how long it will take you to clear security and retrieve your bags. Just be sure to tell them if you only have carry on, or if you have to wait to retrieve checked luggage, as that will take slightly longer.