By default, Microsoft Defender Antivirus will check for an update 15 minutes before the time of any scheduled scans. Enabling these settings will override that default.

Does Windows Defender need to be updated?

Microsoft Defender Antivirus requires monthly updates (KB4052623) known as platform updates. You can manage the distribution of updates through one of the following methods: Windows Server Update Service (WSUS)

Does Windows 10 defender update automatically?

Unlike MSE (and Defender) in Win7, Defender in Win10 (as well as Win8. 1) will ONLY auto-update itself when Windows Update is set to the default Automatic configuration. If you wanna leave it set to Notify Only, you’ll have to update Defender manually.