Excel 2007 :: Start Timer After Workbook Open For Certain Amount Of Time

I have the following code below for a timer in a userform. Right now it is configured to start when a button is clicked. I would like to have it automatically start the timer after the workbook is open for more than 5 minutes but so far have been unable to get it to do so. I have tried putting the code in the workbook module but it still won't run. It also has a button to reset the timer if they need more time in the workbook and also a button for them to save and close the workbook if they are finished. I'm running Excel 2007.

VB:
Private Sub CBReset_Click()
Dim T, E, M As Double, S As Double
T = Timer
Do
E = CDbl(Time) * 24 * 60 * 60 - T 'elapsed time in secs
M = AllowedTime - 1 - Int(E / 60)
S = 59 - Round((E / 60 - Int(E / 60)) * 60, 0)

[Code] .....