I simply do it at least one hour daily. As in every day. I used to be hopeless, my knees made noises, my hips were stiff, my lower back hurt, my shoulder blades used to hurt as if there were real blades in their places. A pathetic walking mannequin.
I started stretching some years ago, but I started getting serious about it in March 2020. I do martial arts and my hips would hurt each time I'd try a high kick or do a sharp turn from the hips. My knees objected after every longer run.
Then, 15 months ago, I stopped sitting on the sofa when watching whatever I watch on TV in the evening and I've been on the floor ever since. If the feature is 2 hours long, I'll stretch for two hours, like a chewing gum. It took me 10 months to do the stuff on the photo. The resulting benefits coming from relaxed hip flexors mean that my lower back does what it couldn't do in 1985, let alone since. I don't turn my torso left/right when running, I go straight. My martial arts stance is straight on, I move with more ease, I go faster. The key is to find out what dynamic and static stretches work best for your body and then work on it diligently. I don't sit on a chair for more than 30 minutes in one go, I'll stand up and stretch for a few minutes. Is that a tad obsessive? Yep. But it's also beneficial. I pick up stuff from the floor without bending my knees. And my knees appreciate it lots.
People often get discouraged in the beginning because stretching exercises are uncomfortable and are bordering on painful and if people don't see any results in weeks, they quit. I ploughed on for 10 months just to get this right (spine up straight, one foot close to the bum, another foot near the opposite knee, both knees on the ground). Once you achieve that, you simply know that other stuff is up for grabs if you are really keen to achieve it.
(btw, where have the people in that photo gone? You can only see some lady's legs further down, a sunbathing aficionado. Those who know me know really well that I love hot weather and this shot was taken by my sun worshiping wife at 2pm on Sunday, after the run, with temperatures being close to 28C. Livin' it, lovin' it!)
In my blogs about stretching and strength & cross training, I go about it for a bit longer. Check it out if you grab a moment.