Is it possible to set up an Excel workbook to timeout and close automatically
after a given amount of time, say 15 minutes? Maybe via a macro that times
out and closes the workbook? Or perhaps a macro from another workbook that
can close it down?

The problem we are having in the Company is that one user will open an Excel
workbook from our network and forget to close it. Then when someone else
tries to open it and make modifications they get the "File in Use" dialog box
which says "filename.xls is locked for editing." and the only buttons
available are "Read Only", "Notify", or "Cancel".