I am not sure about the longest but a round in the 19 cent was measured by knockdowns, so some fights were as long fifty and sixty rounds.Timed rounds came around the turn of the century where 3 min round were established and bouts wold go 15 to 20 rounds.After WWI bouts would last 15 rounds which lasted up to the 1970's then they would only go up to 12.
I'm sure someone can look up the actual dates.
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