What benefits do you gain from stretching and how long do you need to stretch to have a positive effect?

Decreases muscle stiffness and increases range of motion,
May reduce your risk of injury,
Helps relieve post-exercise aches and pains,
Improves posture,
Helps reduce or manage stress,
Reduces muscular tension and enhances muscular relaxation,
Improves mechanical efficiency and overall functional performance. Stretching a muscle to the full extent of your ability and holding it for 15 to 30 seconds is what's called a static stretch, and there's no harm in stretching that way as long you don't stretch until it hurts.