Windows Defender AV issues new definitions every 2 hours, however, you can find more information on definition update control here, here and here.

Is there an update for Windows Defender?

Microsoft Defender Antivirus and other Microsoft antimalware solutions provide a way to manually trigger an update. In Windows 10, select Check for updates in the Windows Security Virus & threat protection screen to check for the latest updates.

How do I update Windows Defender definitions?

Open the Settings app. Go to Update & security -> Windows Update. On the right, click Check for updates. Windows 10 will download and install definitions for Defender (if available).