Service duration is the average time it takes to service an order on-site. You can control this on a customer basis as well, which will override the service duration set in Configuration for this specific customer.
Service duration can be applied task by task, or if you have multiple tasks going to the same place, you can have a set amount of service duration apply to all of these tasks as if they were one.
To learn how global service duration can be set up and how it works, take a look at the Service Duration article.
Adding service duration for a customer can be done in Customers ⇒ Customer Profile ⇒ Location.
Click Save once the information has been added.