You can, as David suggested, query the database for history lines and report on that.
Another way to do it is to set up a date type custom field to store the date it changed to active. Create 3 db rules. 1. set it to null, when the status is anything but stock or active. 2. set it to some invalid value like 01/01/1900 when status is Stock 3. set it to current date when it is 01/01/1900 and status is set to Active.
Then you can create a view and filter by the value of this field.
1. set it to null, when the status is anything but stock or active.
When a CI went from Active to decommissioned or any other statuses after "Active" in the CI lifecycle, then you would lose the history of when it went from stock to active. Perhaps it would be better to have
1. set it to Null when status is set to any of the status's that proceed the "In Stock" status.
This should hardly ever occur but it would mean the history of the CI going to active is not lost.