We do this all the time  with Access databases
(http://support.microsoft.com/?id=128814). I have adapted the same concept for
Excel.

In the ThisWorkbook module of the workbook, paste the following code:

Private Sub Workbook_Open()
'Set StartTime when the workbook  is opened.
StartTime = Timer
'Schedule a call to CheckTime in the future to check elapsed idle time.
Application.OnTime (Now + TimeValue(TimeCheckDelay)), "CheckTime"
End Sub

Private Sub Workbook_SheetChange(ByVal Sh As Object, ByVal Target As Range)
'Something changed in the workbook, so reset StartTime.
StartTime = Timer
End Sub

In A VBA code module in the same workbook, paste this code:

Global StartTime As Single
Global Const TimeLimitInMinutes = 58 'idle time threshold
Global Const TimeCheckDelay = "00:10:00"

Sub CheckTime()
Dim NewTime As Single
'Get the time (seconds past midnight) now.
NewTime = Timer
'If StartTime was yesterday, add 86400 seconds to NewTime.
If NewTime < StartTime Then
NewTime = NewTime + 86400
End If
'If TimeLimitInMinutes has expired since StartTime was last
'updated, close  the workbook without saving changes.
If (NewTime - StartTime) > (TimeLimitInMinutes * 60) Then
ThisWorkbook.Saved = True
ThisWorkbook.Close SaveChanges:=False
Else
'Otherwise, schedule a call to CheckTime in the future to check
'again later.
Application.OnTime (Now + TimeValue(TimeCheckDelay)), "CheckTime"
End If
End Sub

The code above is set to close the workbook without saving changes if it is
idle for more than 58 minutes. It will check every 10 minutes.

You might like to restart the timer on Workbook_SheetSelectionChange events
as well as (or simply instead of) change events. This would allow you to
keep it "alive" simply by clicking a cell, rather than changing it.