For the fellow search engine travelers, there is a work around.
You can set the Alert timeout to 50 days, using a registry setting:
Key: HKEY_CURRENT_USER\Software\Microsoft\Office\<version>\Common\DesktopAlerts
Value name: TimeOn
Value type: REG_DWORD
Value: 4294964295 (Decimal)
If this key isn’t available choose Edit-> New-> DWORD Value and name it TimeOn
(case sensitive!).
On Windows 8 or higher, running Outlook 2013 or higher, Windows instead controls notifications through the action centre. The default notification display time is 5 seconds, but can be made as long as 5 minutes by changing the message duration (in seconds):
Key: HKEY_CURRENT_USER\Control Panel\Accessibility
Value name: MessageDuration
Value type: REG_DWORD
Source : http://www.howto-outlook.com/howto/newmailalert.htm