Contest duration in animals is often interpreted as being a consequence of mutual assessment of the difference in the competitors' resource-holding potential (RHP), allowing the inferior individual to avoid costly interactions it is likely to lose. Duration is thus predicted by the relative size of the competitors, and increases as the difference between them decreases. Alternatively, each individual may persist in accordance with thresholds determined by its own RHP, and weaker rivals retreat because they have lower thresholds. Contest duration depends on the RHP of the contestant that gives up first. Recent work suggests that even though duration is determined by the loser's size, this hypothesis also predicts a negative correlation between duration and the relative RHP of the contestants. However, it predicts (unlike the mutual assessment hypothesis) that contest duration should increase with the mean size of the contestants. We studied the determinants of fighting duration in the fiddler crab Uca mjoebergi. Fight duration increased with increasing size of the loser, and decreased, but to a lesser extent, with increasing size of the winner. Fights between size-matched individuals increased in duration with increasing mean size of the competitors. Neither the mutual assessment nor own-RHP-dependent persistence hypotheses can accurately explain the data. Instead, we present a modification of recent modelling work, and suggest that in U. mjoebergi individual cost thresholds may determine duration, but that larger opponents may inflict those costs more rapidly, consistent with the cumulative assessment game of animal conflict.