sys_journal_field is the table you want, don't recommend it though. This table will be huge and reports likely be quite slow to execute, also it's hard to reference back to Incident tickets as the reference is held by a String document ID rather than a more common reference field.

The better way of handling this requirement would be to use Metrics, which would allow you to track the assignment of a ticket by person and provide the length of time each user held on to it, for instance.

https://docs.servicenow.com/bundle/kingston-performance-analytics-and-reporting/page/use/reporting/t...