There is not a way to see how long a volunteer has been inactive, however you can use other fields in the Rosterizer such as "Last date used in a schedule" and then run that report to determine who has not been used in the software for a very long time.
1. First go to the Rosterizer in MSP, and for "Included Ministries" set this to "Include all ministries"
2. Make sure "Included Volunteers" is set to "Include all volunteers" and that the option at the bottom to "Omit inactive volunteers" is not checked.
3. Then in "Included Fields," under the "Available fields:" column un-check the box that says "Show common fields only," then you can locate the "Last date used in a schedule" field and move that over to the :Fields to include:" column.
4. In "List Arrangement" I recommend choosing the first option to group volunteers into one long list, as well as checking the "List (included) inactive volunteers separately at end of list" option, so you can clearly see who is inactive.
5. In "Document Settings" specify if you would like a Word or Excel document, and then use the "Generate Roster" button to save this on your computer.