There is a school of thought that says that stretching is bad for you. I disagree.
Just when should you stretch?
The best time is after a workout. Your muscles and tendons are warm and will benefit more from the stretch than at any other time. You speed up your recovery process and will also benefit from the relaxation that stretching brings.
Why should you stretch?
You are made up of an amazing intricate set of muscles. These muscles are responsible for all movement. When you exercise you are taking the muscles involved through a series of contractions. After a workout the muscles can remain contracted. If left in this state the muscle will both restrict movement and also trap lactic acid inside it.
One of the marks of growing older is that you begin to lose flexion. The more flexible you keep yourself, the younger you will feel and the better you will be able to perform.
How far do I need to stretch a muscle?
You need to get the muscle to the point where you feel it stretch. This is a stretch session and not a torture session. You do not do yourself a favour by stretching to the point of agony. Less is more.
Your stretch routine should be related to the type of exercise you are doing. Swimmers will need to spend more time on their upper bodies while runners and cyclists will need to spend more time on their legs and back. Triathletes need to get everything stretched out.
Specialist sport shops, Physios and biokineticists should be able to advise you on how to stretch.
http://www.mayoclinic.com/health/stretching/SM00043/
