It is very rare that Symantec issues updated virus defintions on anything less than a daily basis, so once every 24-hours should be enough to keep you current. Empahsys on should...
You can increase the frequency via the Symantec System Console (SSC), assuming you have the NAV Corporate Edition. Access the server you want to change, right click on it, choose Tasks -> Norton Antivirus, and then choose the Virus defintion manager option. From there you have access to all of the controls for the definiton downloads.
Hope that helps - Chad Stiller, MCSE
Ramcom Technology
cds@ramcom.com