Which OS platform?
Clicking sync in the MEM admin console sends a signal to the device through the appropriate notification network. Each of these networks have latency and throttling but in general, for a one-off sync request, assuming the device is connected to the Internet, it should be quite quick, and all changes should be applied.
Normal policy refresh times are listed at https://learn.microsoft.com/en-us/mem/intune/configuration/device-profile-troubleshoot#how-long-does-it-take-for-devices-to-get-a-policy-profile-or-app-after-they-are-assigned