Need code to run macro in excel every 15 minutes, but only if user has File open

I'm working on a project to update production notes in a manufacturing schedule. I've a macro written to pull in the data to the manufacturing schedule, but I need to execute the macro in the file automaticaly every 16 minutes, but only while the user has the manufacturing schedule open.

I've tried using "Application.OnTime" with specific times, but this seems to open the manufacturing schdule if its closed.
A point that may also be relevant is that this will be a shared file on Excel 2010.

I'm pretty new to VBA so any assistance with the code would be much appreciated. The code I'm using is pretty basic:

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[QUOTE=JosephP;3241610]your open event should schedule the macro once only storing the run time in a public variable. the macro should then update the run time for the next interval and reschedule itself. the beforeclose event of the workbook should then cancel the scheduled run using the final parameter of Ontime

Hi JosephP, Thanks so much for your response, this worked brilliantly.