Create a calculated field that can either be
[close date]-[open date]
or datediff('day',[close date],[open date])
You can then use this, setting the aggregation as average, and dragging [open date] to the shelf and setting its aggregation to month.
If you need help following this, include a tbwx with mocked up data (if necessary)
Perfect. Thank you.
(Tami I think you meant Alex gave you the Correct Answer.)