Great question..... I'm honestly not sure. I tend to get distracted fairly easily, so I usually play 2-3 tables to keep my interest. I tend to play in spurts, probably about an hour and a half long on average. I've played 3-4 hours sessions plenty of times though, and I don't think it's affected my results... so I don't know that I can answer your question.
Probably around the 4-5 hour mark for me though. Just a guess.
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