You can gain hamstring flexibility quickly; if you stretch the right way (see video).
In most yoga classes we’re only a few breaths in a pose. That’s often too short if you’re really struggling with inflexible muscles. Too ‘lengthen’ them you have to reprogram your nervous system. There are different ways to do that. A good way is to stay in a pose for a long period of time. The pain you feel when you stretch is actually a signal from your nervous system that tells you to stop stretching, because it’s afraid you will tear your muscles. If you stay in this stretch for a longer period of time (3 till 5 minutes) you nervous system ‘sees’ that it’s okay. That your muscle is not going to tear and the pain will ease. Now you can stretch a little deeper till you feel a slight pain again.
Every time you stretch like this, your nervous system will reprogram itself and make the ‘safety’ marge a little bit less tight. If you do this exercise every day you will gain flexibility in your hamstrings real quick .
Now don’t expect miracles. It will still take time to gain hamstring flexibility, but by long stretches you will gain it a lot quicker than by stretching only for a few breaths.