I want to start stretching. How long should I hold the stretched position?

The best way to stretch is to warm up for at least 5 to 10 minutes before you stretch.  It can be dangerous to stretch a cold muscle.  Once you’re ready to stretch, hold the stretch for 5 seconds, release for 5 seconds, and then execute again, holding for 10-30 seconds.  Repeat two or three times.  And definitely don’t hearken back to the 80s and bounce.  It’s not a good idea.  It can also help to inhale when you’re about to stretch, and then exhale slowly as you stretch.

If you have a muscle that is short and needs to be lengthened, research shows that holding it in the stretched position for 20-40 seconds can increase its length. It is important to understand how to stretch as many people do this wrong. If you are stretching a muscle, you can assume it is shorter than normal length. Move the muscle into a lengthened position to the point where you first feel tension and stop. Pushing it farther is not better. Hold this position for the 20-40 seconds and then slowly release. You can repeat if desired. Injuries can and have happened due to people taking a stretch beyond their normal range of motion. Think of it this way, if you could move it really far, you probably did not need to stretch it in the first place.

There are different types of stretching and flexibility techniques out there. The one most people associate with the word “stretching” is the technique of static stretching. This is where we get into a particular position or posture and hold it for 20-30 seconds. The purpose of static stretching is to lengthen muscle groups that tend to shorten up and become overactive. Improving the motion of overactive muscles increases the flexibility of the joints they pull on, thereby making our ability to move better and more efficient.
Static stretching with the 20-30 second hold is also where most of us should start with stretching. Below is a great example, the calf stretch.
Static calf stretch:

a. place your hands on a wall in front of you
b. take one step forward
c. straighten the back leg and squeeze the butt and quads to keep it locked out
d. be sure your back foot points straight or slightly inward
e. keeping your back heel firmly on the ground, lean toward the wall until you feel a slight stretch in the calf of your back leg
f. hold this position for 20-30 seconds and remember to breathe!