Originally Posted by
Khorianas
I can somewhat agree with that analogy, but it ends where air is indeed spent while breathing. Then again, air is usually also in virtually limitless supply. Sadly we know next to nothing about the finity or infinity of magic we just have the feelings of Rhonin towards it. Maybe the density of magic is used as a measure to the Magnitude or Power of the magic one is able to work. Perhaps it is about "feeling" the presence of magic and redirecting the flow or better holding it together for reshaping it. There most likely is a maximum amount of magic you can keep in one spell with "denser" magic it is easier too keep a big amount together and focus it in one cast. I woul think the "breeting in" or "collecting magic energy" is the tiring process in it. In the end mages have to manage one resource more which they have to attune to. While it is an energy source that leads to massive effects, it is a burden. Also recently in the Tides of War Karlecgos explains to Jaina that magic at it's core is a complex assortment of Formula, so math. There has to be some kind of balance and limit to it.
Also on the core topic "casters tyring" again we don't really have solid evidence of the rate of tiring of casters. We do know that they need to eat and drink a lot to keep casting (Mentioned in every other warcraft book, but maybe also a wink to casters eating and drinking inagame, again difficult.) So it is also physically straining in some way. In the Tides of War, two magi reenforcing a gate with magic are overwhelmed in a period of time I would count in the range of hours if not less. Highly trained mages from Dalaran too. Sadly most of our Novels tell of rather prominent and powerful mages so there is no saying what an everyday battlemage can do.