A minimum flow rate is required to maintain steady Taylor cone-jets issued from an electrified nozzle. This letter reports that the well-known scaling law proportional to the charge relaxation time and independent of the nozzle diameter is only applicable to lowly viscous systems. For highly viscous systems with rapid viscous diffusion, the cone-jet transitional region spans a length scale comparable to the nozzle diameter; the minimum flow rate appears to be proportional to the capillary-viscous velocity and the cross-sectional area of the nozzle, but independent of the liquid conductivity. © 2014 AIP Publishing LLC.