It goes poof without any chromatic flames or anything as those I gifted to would tell me.
It may be code designed to prevent players from cheating actually. Some players would dump gear on NPCS, then go kill the NPC with an alt for example. Also maybe meant to redress that at level 15, a full set of true hero eq can make you almost unkillable.
It does end, but if you deck out a level 11 in legendary gear, when they log off and come back it can be gone without chromatic flames. And it can happen without logging off far as I know, I had a guy tell me gear I gave him disappeared pretty quickly, before he or I had logged out.
I have no idea what the mechanism is, but I don't think it's based on an Imm's active involvement.