The more you do it, the better and more effective it is.
The best way to increase your flexibility is by stretching. It can be done when warming up, in between exercises and at the end of session. But remember, you must warm up your muscles, joints and tendons before any physical activity.
There are two types of stretching – Active and Static.
Active Stretching can be done 2-3 times per day with and overall time of 20-60min. You should aim to include 6-10 exercises that would be repeated 10-15 times per exercise and 5-6 times per session. The best way to stretch is until a feeling of little discomfort.
Static Stretching is longer and much slower. It is very safe and will reduce your risk of injuries. Should be done once per day, around 2-4 time per week. In each session aim to use 60 seconds per muscle group (can be split into 10-30s repetitions).
One of the goals and benefits for passive stretching is the increasesed and improved amount of movement.