It depends on the fatigue. A mage with a high amount of fatigue will cast spells (unless specifically scripted) that won't drop it too far into unconsiousness. So you will rarely see a mage with 90+ fatigue casting anything that would put it at 130+ fatigue unless you either A.) scripted it or B.) the AI decides it wants to for whatever reason (if it's target is really close, sometimes it will cast shards, or if the nearest units are routing, it will try to target farther units, which might move it out of range of other spells)