It's also a subjective thing. What one person considers to be a tender boat may not be to someone else. For example, to my wife anything that heels more than 5 degrees is tender. To some folks, you don't start really sailing until the boat is heeled over 15-20 degrees.
Most designers have a target range for their boats to heel, so some boats will heel more than others in the normal course of things. It's good to know the designer's intent because you don't want to push the boat beyond that amount of heel or the boat will want to round up. To prevent that, you have to turn the wheel to keep the boat going straight and then the rudder starts to act as a brake slowing you down.