There's nothing in that API endpoint that I know of. What I do is build a workflow that pulls this data from my MongoDB instances that way I can get exactly what I want. I also recommend implementing the "Alteryx Server Usage Report" if you haven't seen it yet as it has a lot of great detail on your workflows.
@joshuaburkhow is correct. Currently the API does not report the completion time of a workflow run. Also, as mentioned above, this information can be pulled from your MongoDB instance, on the AlteryxService database and in the AS_Queue and AS_Results tables. To find workflows run through the API, you will need to filter on the IsHidden field (set it to filter on True) in the AS_Queue collection. Please note, if you have the functionality to allow users to run workflows without signing in, these will also be reported. If this is the case, you can also add a filter to match the username to the the SubscriptionID that houses the workflows.
Attached is a sample workflow I use to get Run and Queue times for workflows running through the Gallery that you can use. Just swap out the server name and passwords in the MongoDB Input tools