The way you stretch is almost as important if not more important than how often you do it. You have to stretch ferociously to improve flexibility.
Stretch at any time, all the time, and hard. By hard I mean don't stretch slowly and hold, this will not increase your flexibility when you want it, which is when you're training, rolling, and sparring. You need to stretch to your max point fast and repeatedly, pushing a bit more each time. If you wanna put your foot behind your head, grab your foot at any time during the day and just try and blast it behind your head.
Of course, don't yank once you've reached your max, you don't want to injure yourself. But just remember, if you pulled your muscle out of your body, it would be extremely flexible, by stretching your are training your brain to accept a new movement/position, not actually changing your muscle tissue.