Probably not much. Unless you have a provider which loves bruteforce approach of nat handling (callcentric forces a refresh every 70 seconds or so, resizes the window based on some behaviours), then you can be ok with around 2 packets every 10 minutes. If your connection is already live (email check takes much more traffic), then you shouldn't see much difference in usage.
Gingerbread's built-in SIP client is great; I couldn't see a noticeable difference in battery life running it all day. Before Gingerbread, I ran Skype, which couldn't get through a full day on a single charge. Before Skype I tried SIPDroid, which was a lot better than Skype, but not as nice as the built-in Gingerbread one.
My Skype account had lots of contacts and lots of chats open, so that may have had an adverse effect on battery life given that the communication is all P2P and required keeping an insane number of sockets open.
