Supremely trivial:
This evening, DP made us both a hot drink, sat down, and I joined him on the sofa. He finished the last thirty seconds of the YouTube video he'd been watching, and he said "Right, I'm going to bed in a mo." When I acted surprised, because he'd just made us a hot drink that I'd expect us to take at least ten minutes to drink, he replied, "I said in a mo, not right now!" It's context-dependent, obviously, but to me, "in a mo" usually means "extremely shortly", like maybe less than a minute or two. To him it seems to imply a longer period of time, which I'd probably describe as "soon" or "in a little while" rather than "in a mo".
YABU: Of course "I'm going to bed in a mo" means "I'm going to bed in a quarter of an hour or so when I've finished my drink"
YANBU: Agreed, "I'm going to bed in a mo" means I'm probably going to get up from the sofa in less than a couple of minutes.