Longest time I ever spent grinding a single thing was Atma of the Fish (pre-buffed drop rate). Took me about a total of 15 to 20 hours total over a period of about 5 days to get it iirc. While farming dungeons I'm probably gonna go with doing countless Mythflox and only having seen the Baby Opo-opo drop once and losing the roll to my brother. Still don't have it :/. I also did about 25ish runs for the Demon Gloves of Casting from WoD.