when I qualified as a personal trainer we were told that to increase you're flexibility you had to stretch twice per day. Even once per day wasn't enough and your muscles would return to normal again within a 24 hour period.
Having said that when I used to train only once or twice per week in kickboxing I got more and more flexible as time went by.
A massive source of flexibility knowledge is in the book 'Scientific Stretching' by Thomas Kurtz. You've probably heard that by now but it is true.
Up to twice per day is fine for static stretching (the usual kind where you slide into a stretched position and hold for a time limit of usually between 15 and 30 seconds) but isometric stretching (the single most effective and quickest way to see results - apparently) is similar to weight training in the fact that 3-4 times per week is the maximum as said by vegantkd (isometric being where you slide to near maximum, squeeze your muscles hard to oppose the direction of the stretch for 3-5 seconds and then relax. As you relax you sink a little deeper into the stretch and hold for another 3-5 seconds and repeat accordingly until your maximum is reached-this method relies on the fact that a relaxed muscle is longer than a tensed one, and when you tense a stretched muscle it becomes longer when relaxed).
I suggest this book.
Hope that helps a little.
Train hard and act as if you like it!