How long can you grind?

13-14 hours is probably my max. I don't know, I've never done grind sessions past 13 hours straight. I used to do a lot of 6-8 hour sessions, but then I changed to strictly 4 hour sessions. How often I train goes up and down, often in long bursts followed by long cooling downs. *shrugs*

I just recently farmed Blue Kents for 8 hours straight one day, looking for a Notebook. I've lost track of how many total hours I've put in there. For some reason, even though I'm gaining almost literally no % at a place like that, it doesn't seem tedious. When quests give me a focused goal, I have no trouble committing to it - no matter what it takes.