Stolen from a very nearby universe, er, answer to a question, and modified to
protect the guilty.
Option Explicit
Dim nextRunTime
Sub startRunning()
'you could put
' Run "MyCode"
'in the Workbook_Open() event
'
MyCode
End Sub
Sub stopRunning()
On Error Resume Next
Application.OnTime nextRunTime, "MyCode", , False
End Sub
Sub MyCode()
nextSecond = Now + TimeValue("00:15:00")
Application.OnTime nextSecond, "MyCode"
'your process to run right in with this stuff
'it has already set itself to run again in 15 minutes
End Sub