Lengthening and loosening your muscles could take several weeks or several months.The exact amount of time depends on your degree of tightness, diligence in stretching and your anatomy -- if you were born with short hamstrings, you may never have the same level of flexibility that someone with longer hamstrings has. However, continued improvements should be noticeable each week.
The important thing about full splits is not letting your hips twist. Start from lunge position as in fig 1 and gradually straighten your front leg.
To progress or maintain a stretch such as splits you need to practice regularly. I stretch daily for about 30 minutes doing various stretches. But realistically aim for a session a few times a week, and when you're sitting around watching TV. Warm up first, e.g running up the stairs. Hold each stretch for 10 seconds, do this 3 times and push yourself a little bit further each time.
Dynamic stretching can be done any time, any amount of times, by anyone, with or without warmup (these actually become the warmup). You’re really not stretching, and you’re not going out of your range of motion, so, these are extremely safe. Most important is to do them at the beginning of your day, within 15 minutes of waking up, and then before your workouts.
Static stretching (active or passive) has its purpose. They need to be done when muscles have warmed up. They cannot be done to the exclusion of all other kinds of stretching, because they are not effective by themselves. They are good for when working out “on the mend” - but only when the doctor says so. Some statics (quads, calves) are best done at the end of a workout to help mitigate soreness later on - but these are not meant to improve range of motion, only to relieve injury from soreness.
for more information please visit: https://www.easyflexibility.com/blogs/flexibility-pearls/how-often-and-how-long-to-stretch