Everything JLSeagull said is accurate. Ball valves torque is determined by the seat material, the ball material, the process fluid lubricating (or fouling) the seats, the pressure differential loading the ball against the seat, the design of the seats (some seats are preloaded for application reasons but this causes higher torque). As he suggested, all these factors are considered in the manufacturers' torque tables. Some manufacturers also publish a liat of safety factors for actuator sizing....if the valve sits two weeks between actuation, it will take a set and need a mighty heave to make it start moving.
You did not specify ball valves in your question, though. Butterfly valves also have torque consideretions:
Type of valve, seat material, process fluid velocity and density, packing material all affect the torque required. Again, there is no national standard and there is much variation between similar valves from different nmanufacturers. Consult the manufacturer's literature.