I'll probably get a nice number of downvotes again...but all the above answers are wrong from the beginning to the end. And the answer is already in your question:
there is a CIPA standard for measuring image stabilization
That's all. The concept here is "frame of reference": as there is a standard, there must be a way to test all cameras in the same way and produce a number that is a valid indicator, i.e. it is "comparable" across cameras.
CIPA Test: how it works
(and probably in-house tests before CIPA standardization, too)
As "there is a CIPA standard for measuring image stabilization", 5 stop (e.g.) of stabilization is the result of a standard test that measure under specific conditions how much the camera can be pushed before a certain set of things happens (namely, bokeh degradation and motion blur).
Note: there are -at least- 50 pages in the CIPA image stabilization test procedures manual. And I don't remember them all, nor I've got the brain to understand every aspect of them (even if I produce software for vibrations testing platforms :-D); following explanation is a large oversimplification, if someone wants to go into fine details he can just read the procedure by himself, it's publicy available
The CIPA standard use a vibratory platform to test the camera. That's the magic.
The camera is put on a platform that produce vibrations and aimed to a "standard image"; the platform is powered off and a reference shot is taken. Then the platform is powered on, a set of vibrations are produced, a lot of shots are taken at different shutter speeds, and the moment the camera start producing bad photos is the moment the IS is not able to correct the exposition. Then just imagine that difference between the initial shutter speed and the last good one, expressed in stop, it's the amount of stops the camera stabilization system is able to manage.
Moreover, there is a problem with the question you posed:
it is possible to shoot at 12mm with shutter-speeds of up to 2.6s
and at 100mm with speeds of 1/3s! This is calculated using the
Why is not possible to shot at 100mm with shutter speed longer than 1/3? Simple because you have imposed it yourself in the example! :-)
If you establish that handeld you can shot 100mm at maximum 1/100s, and you then apply 5 stops and it results in 1/3s at max...it's because you made the math, not because the image stabilization system will shut down after 1/3 of a seconds, nor because it will start to work badly after that time! Indeed, image stabilization systems are tested (if I remember correctly) with exposures up to 32 secs :-D
You set the frame of reference here, saying "I take the 1/mm rule and apply the stop factor", so you forced yourself in the corner. What if someone with a really steady hand can shot handeld 100mm@1sec? Does the system stop working after 1/3s even for him because you can't go more than 100mm@1/100th of second?