Although I cannot address your question about how many days would be sufficient for you to set as the retention period I can assist you with updating the parameter.
Once you have made a decision on what to set the retention period as from a SYSVIEW session enter the commands:
LIBVIEW PARMLIB CAPtURE.
Take note of the library where the member is coming from.
Get into an Edit session against that library and the CAPTURE member itself.
Update the AllocRetention to reflect the new number of days to retain the data.
Save the change.
Again from a SYSVIEW session enter the command:
Do a Find on CAPTURE
If the member is there issue the LineCmd
Once it has been reloaded Select the member.
Does it have the new AllocRetention period?
If so then from the SYSVIEW session enter the command:
Issue STOP against the SYSVUSER CAPTURE subtask.
Once the task has become INACTIVE issue START against the SYSVUSER CAPTURE subtask.
Get out of your SYSVIEW session and start a new session.
Once you are back into SYSVIEW issue the CAPPARMS command.
It should reflect the new Retention period.
One additional note:
If you use SITE libraries in your SystemConfiguration options member, put the UPDATED CAPTURE Parmlib member into that SITE library.