If we really want to lengthen tighter muscles we should be holding our stretches for at least 30 seconds. And we should be repeating these stretches approx 4-8 times per day and if not more. Why? To lengthen tissue we need to stimulate our nervous system's proprioceptive counterparts (muscle, tendon and joint cells that tell our nervous system where we are in open space) to report new positive information regarding the muscle's new length and the joint it crosses's new range of motion. So for example, if you loosen of your quadriceps (front thigh muscles) - you can now bend your knee further and your hamstrings can contract closer to maximal.
Basically, we need high repetition to 're-educate' the tissue by 'bugging' the nervous system. You can get as technical as basic you like, it is what it is. We need to "bug our brains" to lengthen tight muscles. Fascinating.
Thanks for reading