It is best to never delete a task. ServiceNow acts as the store of all requests, and it is very useful to have a full audit trail of information. However, closed tasks that are more than a few months old are likely less important, and you certainly don't need them in a shift handover report. Removing them from the primary table will let the database work quicker, because it doesn't need to consider them during queries.
The System Archiving functionality lets you set a condition for a particular table. Most of the time this is based on the active flag and a date range. If the record is inactive and hasn't been edited for a year, it could be a candidate for archiving.
Create entries by navigating to System Archiving > Archive ...
