It is really hard to remember what level you were when you got a certain spell. Everything disappears from your Professor's training menu once you put it in your spell book so even the ORDER of their spells is hazy.
As it has been over 24 hours and no one has responded it looks like most folks have the same problem I do ... ;)
I can tell you I was in Grizzleheim the first time I used it. So I was most likely greater than Level 20. I also remember going BACK to the Tomb of Storms in Krokotopia AFTER I got it to see if it would make a difference against the Djal Quest (4v1). It didn't. I simply did not have enough health to kill enough of those guys to last long enough to cast a minion.
I think I got it AFTER starting Marleybone but before starting Mooshu, but I honestly cannot remember. I know my answer is NOT what you asked for, but no one else is responding, so it's the best I can do.
Please let us know when you get it so we can make note of it.