Viscosity is measured in mPa·s (equivalent to centipoise) on a Brookfield viscometer, or in seconds on a flow cup — Zahn or Ford — in printing environments.
Every process has a viscosity window. Gravure inks run around 15-25 seconds Zahn; flexo similar; roller coating and spray each have their own. Outside the window you get defects: too high and the coating will not level or release from gravure cells, too low and you get poor coverage, flooding and mottle.
Two complications matter in practice. Temperature: viscosity falls sharply as temperature rises, so a press running warm behaves differently from a cold start. And shear-thinning: most coatings are non-Newtonian, meaning viscosity at high shear on press differs from the static measurement in the lab.
During a long run, solvent evaporation raises viscosity progressively, which is why press operators monitor and correct it continuously.