Critical Mass is a wizard passive: Critical Hits have a chance to reduce the cooldown of your spells by 1 second.
Do you know what is meant by "chance" ? It is probably something like an increasing chance over time that decrease every time it triggers, because if it was a flat % chance it would be in the description.
Most wizards cooldowns are about 15 seconds. When compared to evocation (reduces all cooldowns by 15%), critical mass needs to trigger at least 3 times every 15 seconds to be better than evocation. In my current setup, this is never the case. I would say that I deal about 10 critical hits in a normal 15 seconds fight. It is most of the time 1, sometimes 2. So I would guess that the base probability is something like 10%. My critical chance when doing this was 7%, so I think you have to increase it by a lot in order for critical mass to be effective.
Update: Critical mass recently got patched:
Fixed an issue with certain powers that had a higher than intended chance to trigger Critical Mass
It is still not clear how it works. Apparently it depends in what skill you are using. The only way to be sure is actually testing in the game what the skill does; and I find it very frustrating. I want to be able to design a build without having to test myself every spell independently, or searching though the web if someone did it.
Update (1.0.3): With the attack speed nerf, critical mass got un-nerfed. There is a build called windup based on the fact that energy twister with wicked wind makes critical mass trigger very often.
Update (1.0.4): Now the skill energy twister - wicked wind used in the windup build has his modifier reduced by half, so the build is still good, but not as good as it used to be.