"Ten percent luck, twenty percent skill, fifteen percent concentrated power of will"- Remember the Name, by Fort Minor
In Kale, magic is widespread and often barely controlled. No ritual is required to use magic. It's pretty much all willpower. The main limiting factor is physical. Using magic has a bit of backlash, and some of the magic enters the bloodstream in its pure form. At this point, things can go several ways. At low levels, this energy is actually beneficial, and while in circulation it improves the reflexes, strength, and natural healing of the mage. Most soldiers can use at least a little bit of magic, both to protect them from other mages and to harness this effect.
If more magic is used, however, things become a little different. An experienced mage can handle a lot more magic in the bloodstream than a civilian, since a sort of "immunity" is developed over time. If the body's limit is exceeded, then bad things start to happen. Fatigue sets in, and the initial positive effects begin to fade. If the mage continues using magic past this point, they may collapse or fall unconscious. In the worst case, if too much magic becomes present in the bloodstream too fast, it can actually begin to burn away the blood. In this case, the mage's power will increase dramatically, and as long as they continue using magic they're fine. Eventually, though, the mage's reserves will run out, and they will become unable to support their unstable state. They will die. Once a mage's blood is burned away, he is effectively a dead man walking, though his temporary power often allows him to complete one final task.
The greatest mages eventually end up with magic as a constant part of their system. They stop aging, and will not die of natural causes. This immortality, however, rarely lasts long. If the mage stops fighting they will lose the condition and become mortal again. On the other hand, if the mage keeps fighting, they will eventually be killed in battle. This is the eventual fate of most mages, one way or another.