Steppers are rated at their holding torque, when not spinning. the faster they spin, the less torque they have. But, this depends on quite a few things. First, you have to supply them with their rated current to get the full rated torque. And the higher the voltage you supply, the longer it will take for the torque will drop off. Ideally, you'd like to find some torque curve charts from the manufacturer, which will give you a good idea of how much torque you will have at a given rpm, based on a certain voltage.