synctime

The default sync frequency is 30 minutes, I assume you are using the latest Azure AD connect on premise. You can change this or even run a forced delta sync.

To change the default sync interval, on the server running AAD Connect

Import-Module ADSync

and then to view current config

Get-ADSyncScheduler 

to customize the sync schedule:

Set-ADSyncScheduler -CustomizedSyncCycleInterval d.HH:mm:ss

to force a Delta sync after you changed something on on-prem AD

Start-ADSyncSyncCycle -PolicyType Delta

More on this documented here:

https://docs.microsoft.com/en-us/azure/active-directory/connect/active-directory-aadconnectsync-feature-scheduler