Hello, I'm a yoga instructor and personal trainer. Just joined the forum.
I agree with all that was advised here. What worked for me is holding the stretches for 1 minute, 5 times on each stretch. On the 3rd stretch is when I start noticing improvements and I can do deeper, that's why I do two more stretches.
The time you hold the stretch and how often you train are not the only important factors. The topic is really extensive but I wanted to add that, joint mobility and massage are key to your flexibility training. Paul Zaichink explains ball massage and that helped me immensely.
Then strength is important, you cannot stretch a weak muscle. Think of a rubber band, a weak one and a strong one, which one can you stretch more before it breaks? :-D And also, when it comes to strengthening the muscle for stretching, you have to take into account the range of motion in which you train. You should train in an extended range of motion, that is, you stretch the muscle and contract it using let's say, rubber bands as resistance. This way the nervous system will get used to the muscle being contracted when stretched. This way you'll be flexible and strong. This has many similarities with the PNF method I believe.
Then, it also depends on what you want to do with a split, if you just want to hold it for the picture and the ladies then you train one way, it does not mean you'll be able to kick. Or if you are a martial artists you may train in a different way, less static stretches and more dynamic (active and passive stretches).
Complex topic :-D If you have any questions, I'm happy to share what I know.
How long have you been training for split? Keep us posted on your progress.