I would say: 8GB! :)
Joking aside: The camera does not save continuously to the SD card, it depends on how often something was recognised and what you want to have recognised, because only that is saved. If nothing is recognised, nothing is saved!
I save about 18GB per month, with 24 hours of activity, per day there are about 120 recordings of 3 to 6 MB each. A single video of 1 minute takes about 6 MB. I also have my videos saved on my NAS with ftp, so it doesn't matter to me how much space is left on the camera and the videos are overwritten.
I hope this helps!

PS: I wouldn't buy the Presence again though, I would have liked more configuration options.

Wow thanks for the detailed answer, which camera would you rather recommend? :D