How many times a week do we need to do flexibility exercises? For what duration each time?Thanks for the A2A, Sed.
Stretching, when referring to passive or static stretching, should be done after a workout, never before. On the other hand, dynamic stretching is excellent and an important part of a pre-workout routine.
As a rule of thumb, even if you don't exercise every day, static stretching should be done once a day for best results.
As for the duration, a single stretch passive can be held anywhere between 10-30 seconds, for 1-3 reps per stretch. Assuming you are very thorough, this can go for up to 15 minutes and perhaps even longer.
Upon Sed's request, I'd like to add this as an afterthought:
Passive or static stretching before a workout has some detrimental effects, as newer research shows. It slows down blood flow in the stretched muscle(s) , it decreases it's sensitivity to innervation and, effectively, as a consequence, lowers the output of the muscle. Basically, it's like you never warmed up in the first place. Sprints coaches (the good ones!) have been speaking out against passive stretching since they found out, but still, few people know this. It doesn't make much of a difference when you go for a jog, because that is a low-intensity exercise. Anything in the medium or higher intensity range should be preceded by a proper dynamic stretching routine. Dynamic stretching increases blood flow and responsiveness, and is also great as short-term remover of stiffness in a muscle. Static stretching is important for general mobility and increases in range of motion.
To do a static stretch, follow the procedure I outlined above. For a dynamic stretch, you need stretch the group only for a short moment (less than 2 seconds) ad release. The video on the linked page shows some examples of dynamic stretches:
A Dynamic Routine