Using Excel 2000

I am unsuccessfully trying to create a time restriction on an excel file so
that if the workbook has been open for longer than 20 minutes, any changes to
the file will be saved and then the file will automatically be closed. I
would like a message box to be shown when the file is opened, stating that
the file will automatically close in 20 minutes and then have message boxes
shown at the 10, 15 and 18 minute marks as reminders the file will close.

I have tried using the Workbook_Open() event, OnTime Method and a MsgBox but
I have very limited understanding of VB and have not been able to put these
things together in a manner that will produce any results.

Any help is appreciated.