In RAW only mode Canon list a maximum burst of 13 shots and then show in brackets 18 for a UDMA 7 card. I guess this implies a buffer of close to 13 shots with a slow card and it can do better with a faster card. My largest 5D2 RAW file having a glance at one folder is 30MB, assuming that was the average for a 5D3 with a bit more resolution you'd be looking at the following once the buffer was full to empty it: