After a lot of thought and some time spent examining staffing on games I would consider "stable" vs. games that I would consider "unstable fecal matter," I've decided to revise my personal set of staffing guidelines for projects.
All major projects will most likely require a minimum of one experienced test lead and one full-time experienced tester.
Add one inexperienced/contingent tester or one-half of an experienced tester (either locally or at the publisher) for each hour it will take the average gamer to complete all levels and all required and optional objectives/goals.
If the game is multiplayer, add one experienced full-time tester.
If the game's multiplayer mode includes AI, add one experienced full-time tester.
If the game's multiplayer mode involves cooperative play, add four experienced full-time testers.
If the game is shipping on multiple platforms, but sharing a code base, add one experienced full-time tester per platform to focus on certification requirements.
If the game is shipping using a 3D engine that has not been used in a shipping title before, add one experienced full-time tester.
If the team has never shipped a title before, double the testing estimate.