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About 42 hours. I start getting hallucinatory sparkles at roughly 40 hours and usually go to bed then.
Only done it a few times in my life, but the most memorable one was while in the middle of a 5-day LARP. We were going hard, I was NPCing, and I started seeing shadows in the middle of a fight. I took that as my cue to dip out and crash.
Tossup between how much caffeine I drink and not working out.