This is a question that has just about as many answers as the times it has been asked. Seems like the answer changes every day. For ease of reading and understanding I have broken this answer into several parts. Please feel free to contribute.
Part I: Understanding The Components of Muscles
To get the best understanding of how long we should stretch for we need to first understand the components that make up muscles. If you look at a muscle you have the muscle belly sandwiched between a tendon on either side. The muscles bellies are primarily composed of muscle fibers while tendons are primarily composed of collagen fibers. Both the muscle belly and tendons are surrounded and connected by what is called connective tissue. The different components of muscles are affected differently by stretch/load.
Changes in tissue can be categorized into plastic and elastic deformations. Plastic deformation aka creep, is defined as elongation that occurs even after the load is removed. Elastic deformation is defined as spring-like action in which any lengthening of the connective tissue that occurs during stretching is recovered when the load is removed.
Muscle fibers only have elastic properties. Tendons and connective tissue have both elastic and plastic properties. Ordinary stretching will primarily affect the elastic properties of muscular fibers however will do little to the collagenous barriers of tendons and connective tissue.
Stretching should be designed to induced creep if permanent increase in range of motion (ROM) is the goal. Which I think it is for everyone reading this article.
Continue reading this article next week in Part II of this series.