AD Sync can be run manually or can be scheduled to run at set times. If you choose to schedule it please remember that the scheduling and the running of the application in an automatic manner needs to be done from a third-party application. The AD Sync does not include scheduling itself. 


Windows Task Scheduler is usually the tool that is used to schedule this application to run. 



References for Windows Scheduled Tasks


Microsoft "Task Scheduler"


Microsoft Technet "Task Scheduler How To.."


How to Open Windows 8 or 8.1 Task Scheduler


Working with Windows Server 2008 Task Scheduler