I think this can help you :

The Deactivate event occurs when the form or report loses the focus to a
Table, Query, Form, Report, Macro, or Module window, or to the Database
window.

The OnDeactivate value will be one of the following, depending on the
selection chosen in the Choose Builder window (accessed by clicking the Build
button next to the On Deactivate box in the form or report's Properties
window):

- Raoul

The form's Resize event will fire whenever the size of the form changes,
including when it is minimized.  So all you have to do is check, in that
event, to see if the form is minimized.  You can call the "IsIconic"
Windows API function to determine this.  So you might hjave code like
this in your form's module.

'----- start of code -----
Option Compare Database
Option Explicit

Const conInterval = 1000   ' every second
'*** SET YOUR DESIRED TIME INTERVAL HERE

Private Declare Function apiIsIconic _
       Lib "user32" Alias "IsIconic" (ByVal hwnd As Long) As Long

Private Sub Form_Resize()

   If apiIsIconic(Me.hwnd) Then
       Me.TimerInterval = 0
   Else
       If Me.TimerInterval = 0 Then
           Me.TimerInterval = conInterval
       End If
   End If

End Sub

Private Sub Form_Timer()

   ' your timer code here ...

End Sub

'----- end of code -----