How long should a ramp be for a 15 inch rise?

Could you please elaborate on the specifics of the ramp you're referring to? Is it for a wheelchair access ramp, a loading dock, or some other application? The length of the ramp required for a 15-inch rise will depend on the slope gradient, which is typically regulated by safety standards. For example, in many jurisdictions, wheelchair ramps must have a slope no steeper than 1:12, meaning the ramp would need to be at least 15 feet long to accommodate a 15-inch rise. However, if a steeper slope is allowed, the ramp could be shorter. Could you provide more context so I can give you a more accurate answer?