Thumbs up to the above posts
As the others have mentioned, there's no specific bandwidth requirement for the SEPM to the Internet, as there is no constant activity. The majority of any SEPM -> Internet traffic will be definition downloads. The schedule is something you manage of course, the size of the content can be guess-timated from the full def downloads available from the below:
http://www.symantec.com/security_response/definitions.jsp
Add a (very) little extra for the Licensing, Threatcon, and "Defs from Symantec" updates when on the console and you're done.
Don't forget however, that SEP clients also connect directly out to the internet for Insight lookups and anonymous data submissions. More info on the amount of network load this generates can be found in the Insight FAQ below:
http://www.symantec.com/docs/TECH183109