If you go to HKEY_CURRENT_USER\Control Panel\Accessibility in the registry editor, you can manually adjust the "MessageDuration" entry to be whatever you like. However, as far as I can tell (and from what I've seen in articles online), Windows 10 will continue to show notifications for 5 seconds if that entry is set to anything less than 5. It works fine if you choose any number of seconds above 5.