You can use a timer in VBA to get your code running periodycally (for example, every N hours). Take a look at the Outlook VBA - Run a code every half an hour similar thread for more information and sample code.
Also you may find the page helpful.
Place the following code in the ThisOutlookSession module (Tools->Macros->VB Editor):
Private Sub Application_Quit()
If TimerID <> 0 Then Call DeactivateTimer 'Turn off timer upon quitting **VERY IMPORTANT**
End Sub
Private Sub Application_Startup()
MsgBox "Activating the Timer."
Call ActivateTimer(1) 'Set timer to go off every 1 minute
End Sub
Place the following code in an new VBA module
Declare Function SetTimer Lib "user32" (ByVal hwnd As Long, ByVal nIDEvent As Long, ByVal uElapse As Long, ByVal lpTimerfunc As Long) As Long
Declare Function KillTimer Lib "user32" (ByVal hwnd As Long, ByVal nIDEvent As Long) As Long
Public TimerID As Long 'Need a timer ID to eventually turn off the timer.If the timer ID <> 0 then the timer is running
Public Sub ActivateTimer(ByVal nMinutes As Long)
nMinutes = nMinutes * 1000 * 60 'The SetTimer call accepts milliseconds, so convert to minutes
If TimerID <> 0 Then Call DeactivateTimer 'Check to see if timer is running before call to SetTimer
TimerID = SetTimer(0, 0, nMinutes, AddressOf TriggerTimer)
If TimerID = 0 Then
MsgBox "The timer failed to activate."
End If
End Sub
Public Sub DeactivateTimer()
Dim lSuccess As Long
lSuccess = KillTimer(0, TimerID)
If lSuccess = 0 Then
MsgBox "The timer failed to deactivate."
Else
TimerID = 0
End If
End Sub
Public Sub TriggerTimer(ByVal hwnd As Long, ByVal uMsg As Long, ByVal idevent As Long, ByVal Systime As Long)
MsgBox "The TriggerTimer function has been automatically called!"
End Sub