I don’t know right now, but I’ll try and see if I can find out something as that’s something I’d like to know as well.
Incidentally, while searching I stumbled upon a different solution to your problem, which I like rather better than mine:
“I setup a blueprint to count idle time by accumulating Delta Time X in the AnimBlueprint event graph anytime when speed is zero. After the idle time reaches a certain number of seconds I set a boolean value that activates a node in the AnimGraph and I reset the idle time to start counting again.”
(posted by Waves here: How to play random multiple idle animations using montage - Character & Animation - Epic Developer Community Forums)