Microsoft's only on-the-record guidance on how Achievements are approved came more than two years ago, and the maximums it set then have been easily surpassed. Xbox360Achievements.org says it's learned of new policies on Gamerscore ceilings.
X360A's Alan Pettit noted that the new batch of Fallout 3 achievements broke, once again, the stated maximum of 1250 Gamerscore (per title) Microsoft set forth in 2007. His complaint about the inconsistency brought him a response, he says, from an unidentified someone inside Microsoft. This person spelled out two revisions to the Gamerscore/Achievement maximums.
According to X360A, they are:
• A maximum of 1750 points and 80 Achievements for a full retail title. The Orange Box's 99 achievements seem to be grandfathered in, although it's an open question if they'll be allowed to add more achievements should a Team Fortress 2 update come to Xbox 360.
•A quarterly maximum of 250 points and 10 Achievements for any additional DLC associated with that title. Additionally, these totals are cumulative, so a company could wait three quarters to do a large expansion with 30 achievements and 350 points, or parcel them out in smaller additions. X360A says its source explained the rule is meant "to stop companies from releasing a whole slew of content immediately after a game's release to hit its 1750 max right away."
The source had no answers if there were rules regarding rumors "no achievements for free content," or requiring other peripherals or titles to unlock certain achievements. Still, this info, if legit, sheds a little light on how Microsoft tunes its policies with the goal of sustaining interest in a game.
The Current Achievement Rules Explained [Xbox360Achievements.org, thanks Heape.]