The Axsy Field Service Mobile App supports Salesforce's Page Layout customisation. This means that it is possible to add and remove fields that are made available from Salesforce in the Axsy app. 

You can add the Duration field to the Information section of a Work Order so that it will display under the Details tab of the Axsy app. 

Please see this Salesforce Page for more information on how to configure Salesforce's Page Layouts: Page Layouts