Hi Sonja,
The amount of time you keep workitems in your system depends on your legal/business needs. For example, if you are audited and need to be able to show workflow approval steps, you need to either capture this information in a pdf and attach it to the relevant transaction, or keep the workitem information. Once you archive workitems, it is not very easy to read the archived data and show workflow approval.
Best Regards,
Karin Tillotson