I have played a few games with a pretty much constant warfare, and in these I got quite a lot of promotions. I used leaders and civs that give free promotions as well, as I was going for the promotion challenges. I got the one that requires three maxed out trees on one commander, so apparently, I had a level 18 commander in that game. I remember that it was quite painful to get there though, but don't remember which leader/civ combo I used, nor whether it was always full blown war or also pillaging / CS harassment to get there.
I usually fight at least one war per era though, even if playing "peaceful" or "friendly". I seem to get attacked in most games anyway. So, it feels normal to have 2 reasonably good commanders (= 1 commendation in the end, usually +5 CS), but I don't think these two commanders usually get higher than level 6-8.
Maybe of interest to some: for the challenges that require to max out trees, you can also have naval and air commanders, which have less points per tree. I think especially naval commanders are very easy to level up, and I got the challenge that requires three commanders with two maxed out trees with 2 naval commander next to my "main" land commander.