Yeah, this is always difficult to judge
The fact of the matter is that the amount used for the heartbeats and definitions is going to be different everyday, and will depend on (amongst other things) the client activity as well as the number of new signatures added by Symantec.
As far as the bandwidth throttling options between the GUP and the SEPM goes however, try to ensure this is high enough to allow the GUP to download a single copy of the full defs within a reasonable time frame (just in case a client ever needs the full defs over the deltas).
Some sizing info can be found in the below article:
http://www.symantec.com/docs/DOC4448