In SEPM, you cannot define the bandwidth usage.
But you can check the following:
1. You can change the communication mode between the client and sepm as pull mode and set heartbeat interval to 30 mins
http://www.symantec.com/business/support/index?page=content&id=TECH94711&actp=search&viewlocale=en_US&searchid=1294218179671
2. Change the number of content revisions to keep
100 clients: 3
200-500 clients: 10
http://www.symantec.com/business/support/index?page=content&id=TECH92225&actp=search&viewlocale=en_US&searchid=1294218002741