I won't make a solid rule, but I trust players to gauge the power level of what they're doing, and react accordingly.  I see no reason to inflict punishments that can simply be done via roleplaying, if that makes sense.  The general rule of thumb is that the more consecutive uses of a spell you use, or the more powerful the spell/effect is, the more it will tire you. Do it enough times in a short period of time, and you<em>will</em>end up going unconscious, not to mention you'd be introducing a very large amount of Aether into your bloodstream.