The average length of threadlike micelles in a semidilute solution has been inferred from the ratio of loss to storage modulus at a local minimum in loss modulus, which occurs at frequencies well above terminal. This method has yielded average micelle lengths of less than 1 mu m or so for cetyltrimethylammonium surfactants at concentrations of around 0.1 to 1 M [Khatory , Langmuir 9, 1456-1464 (1993)]. Here, I use established correlations for polymer semidilute solutions to show that this estimate of micelle length is likely too small by around an order of magnitude. More accurate estimates can be obtained from the terminal relaxation time and the viscosity near the cross-over from dilute to semidilute, which show typical micelle average lengths that are well above 1 mu m in length, for cetyltrimethylammonium solutions. (C) 2012 The Society of Rheology. [http://dx.doi.org/10.1122/1.4739066]