What's the average power level ("rank", p.55 MtAw) of mages in Mage: The Awakening?
A typical build for a newbie mage is 3 in one arcanum, 2 in another, and 1 in a third. So this is probably a good baseline for "common mage with a little experience or a little talent, but nothing special." Let's say that's the 25th percentile.
Looking at the NPCs in the M:tA book, their highest arcana are 3, 4, 3, 4, and 5, but most have several arcana at or near that level, too. They seem to favor broad skills over maxing out one or two arcana. These are supposed to be major figures in a major city, so I'd take that to mean they're near the top of the pyramid, around the 95th percentile.
So if the median is a little higher than a newly-minted mage but closer to that than to the sample NPC, a typical mage is probably 3, 2, 2, 1; 3, 3, 1; or 4, 2.
The real answer for NPCs that the players actually encounter is "a little higher than any PC." If an NPC can't do anything the PCs can't do for themselves, they're probably not going to be interested in him as an ally. If an antagonist isn't more powerful than each PC alone, she won't be much of a threat for the group. In general; a clever storyteller can make a mundane human a very dangerous opponent.