The first time you run a manual scan there may be a few files sent over the web for checking but subsequently there should only be new ones making repeat scans much quicker. The only files sent for analysis will be 'unknowns'. Bandwidth usage is minimal. I would guess less than an AV downloading definition updates once or twice a day.
Also: Just grabbed this paragraph off CNet.
IanAs light on resources as advertised, Cloud Antivirus offers strong reputation-based protection for those who want their security program out of sight and out of mind. A third-party efficacy evaluation wasn't available at the time of writing, but in empirical testing the program only used 9 MB of RAM while idle, and only 56 MB of RAM when scanning. Many other security programs will run scans at 150 MB of RAM or more.

