Open Start and type "regedit." Alternatively, you can type "regedit" into the search function on your toolbar.
Use the left toolbar to navigate to the "Current Version" folder in the Registry.
Create a new Key under Current Version and label it "Quiet Hours."
Right click on the "Quiet Hours" Key you've just created, and click New > DWORD (32 bit) Value. Name the first value "EntryTime." Create a second DWORD Value and name it "ExitTime."
EntryTime will dictate the start of your quiet hours, while ExitTime will dictate the end of your quiet hours.
Double click "EntryTime" to edit it, and make sure the "decimal" option is selected. In the value data box, enter the number of minutes after midnight you'd like your quiet hours to start (i.e. for 2am you'll enter 120, for 3am > 180, and so on).
Double click "ExitTime" and make the same edits, once again noting how many minutes after midnight you'd like your quiet hours to end (8am=480, 9am=540, etc.).
Once you've entered the start and end times for your quiet hours, you're free to close the Registry Editor! These changes will happen automatically, there's no need to restart your PC.
Congratulations! You've now manually set your quiet hours to temporarily mute all notifications.