It would be difficult to get over 1M on FPS's alone. Simple math at 1 kill every 6 seconds (decent average of multiplayer wait/singleplayer slaughter) would clock in at 1666 hours of pure gameplay (at the 1 kill every 6 seconds) for every million. Unless you are a professional FPS player it is not likely FPS's will get you much more then 1-3 Million. RPG's have the same issue, while when you do kill you can kill alot, that tends to be in bursts so the average, like FPS's won't get much lower then 1 kill every 6 second (and MMOs generally require you to spend a few seconds on a mob, factor in time to gather a group or move to a new target and you are looking at 5 secs a kill max).
Now RTS, especially the ones where units are in clusters and act as cannon fodder, it is much easier to beat the 1 kill every 6 second (1 cluster every 6 second, with a cluster having 5-10 units already makes the total explode).
I put mine at a modest 1-10mil, though I have been a strong RTS fan, I haven't played as strongly the massive unit ones (Shogan Total War being the largest for group clusters) so I have toned it down. Though by the numbers I am probably pushing the 10mil mark.
Good luck on the totaling up ^^.