How much more flexible does daily stretching make you?

Short answer

In 12 weeks, the angle you can raise a straight leg grows by about 15 degrees. You do not need to do it every day. About 10 minutes a week per area gave the full effect, and 2-3 days a week kept it.

How much more flexible does daily stretching make you?
'Stretch every day, hold it longer → get more flexible' — is that right?
A common belief is that flexibility only comes from stretching every day and holding it long. So how much does it actually improve, in how many weeks, and does daily stretching add more? We also checked where the gain comes from and how long it lasts.
How much in how many weeks?
In 12 weeks, the leg-raise angle grew about 15 degrees
173 people stretched the back of the thigh 3 days a week, 3 minutes a session, for 12 weeks. The angle at which they could raise a straight leg lying down grew by 15.14 degrees on average. The group that did not stretch stayed the same. 32 people who stretched daily for 4 weeks went from about 71 to about 90 degrees.
Does stretching more often and longer add more?
Once for 30 seconds equaled 3 times for 60 seconds
93 people stretched the back of the thigh 5 days a week for 6 weeks. 30 vs 60 seconds and 1 vs 3 times a day made no difference. An analysis that pooled 189 studies and 6,654 people found that sessions per week did not change results, and going beyond 10 minutes a week in total added nothing.
But the muscle did not get longer
You just got better at tolerating the pull
60 people stretched 30 minutes every day for 6 weeks, and their leg-raise angle grew 10 degrees. But when pulled with the same force, the angle was no different. An analysis of 65 studies also found muscle fiber length unchanged; tolerance for the pull rose a lot and muscle stiffness fell a little.
Who gains the most?
Stiffer people and women gained more
In the 6,654-person analysis, people who started less flexible gained more. An analysis of 77 studies found women gained more than men, and holding a stretch still worked better than bouncing moves. Age and training experience did not change results.
Do you stiffen up again if you stop?
A little, but you stay above where you started
An analysis of 556 people (13 studies) found that after 5-15 weeks of stretching, 2-6 weeks off lowered flexibility a little, but it stayed above the starting level. People who stretched daily for 4 weeks and then stopped dropped from about 91 to 84 degrees; those who kept going 2-3 days a week did not drop.
Bottom line: not daily, but 10 minutes a week
About 15 degrees in 12 weeks, keep it with 2-3 days a week
Regular stretching adds about 15 degrees to your leg-raise angle in 12 weeks. You do not need to do it daily: about 10 minutes a week per area (4 holds of 30 seconds, 5 days a week) gave the full effect. Once you reach the range you want, 2-3 days a week keeps it.