Hi,
File-based AV - Updated daily (typically three times a day Monday through Friday; one time a day on weekends).
Network-Based Protection (IPS)- Updated on Weekdays (M-F - no releases scheduled for weekend or major holidays in the United States).
Behavior-Based Protection / Proactive Threat Protection - There is no set schedule.
Reputation-Based Protection (Insight) - Continually updated as it is a cloud based detection.
Reference:http://www.symantec.com/docs/TECH178271
We will always suggest to use most latest definitions however in manual update it would be difficult to implement. I would suggest to update once in a day. if there is any alert about new threat from Symantec then need to update definitions immediately.