It's not possible to set a schedule for updating the clients from the SEPM. It's only possible to create a schedule for updating the clients from an internal or external LiveUpdate server.
However, if you want to update the clients from their SEPMs, the best way to minimize network traffic is to prevent full downloads of antivirus content (~ 160 MB). This can be done by storing enough content revisions at the SEPM.
If you are updating your SEPM every 4 hours, for example you could save 21 revisions to cover a complete week. Symantec is sending up to 3 content revisions per day. If you are updating your SEPM once a day, 7 content revisions are enough for a week. Now every client with content not older than a week will pull so-called delta files from its SEPM. These delta files are often not bigger than some 100 KB, compared to 160 MB of a full download.
Here is some stuff to this topic:
With default LiveUpdate content revision settings configured within the Symantec Endpoint Protection Manager, clients are downloading full definition updates instead of delta updates
http://www.symantec.com/docs/TECH94916
Unfortunately, content revisions are eating up a lot of disk space. One content revision has a size of about 900 MB (and this number is constantly growing). See Paul Murgatroyd's post here (and substitue 81.5 by 160):
https://www-secure.symantec.com/connect/forums/disk-space-management-downloads#comment-4211241
Additionally, set the Communication settings (Clients > Policies > Communication settings) to Pull mode, take a generous heartbeat interval and activate the Download Randomization.