I want to increase flexibility in my legs for high kicks (splits, etc.) How long should I hold each stretch for? I've heard a lot of different approaches to this - does anybody know? Does stretching for too long actually harm you? How often should I stretch?

I would suggest a couple of different stretching methods.

In order to do "high kicks", doing dynamic flexibility exercises
is very helpful. I find doing them first thing in the morning,
while difficult, is the most effective. You can do these every
day, or nearly every day, as long as you aren't kicking so high
that you are tearing your muscles. I've had the best success
from doing a few sets of dynamic leg kicks, followed by a
similar static stretch, like putting your foot up on a bar and
stretching. I found that it works best to repeat this sequence
several times.

I also had great success with PNF stretching, and active-
isolated stretching. Both of these methods require you to
follow a specific protocol, and are easier with a partner.
However, they also both lead to fast flexibility gains. I can't do
this more than two or three times a week as it makes my legs
pretty sore.

I went from being an average unflexible guy to getting the
center splits in about nine months through doing PNF and A-I.
After that I learned about dynamic flexibility exercises, and
now mostly do those. I can pretty much go into the center
splits now at any time without being warmed up.

"where do you find these pnf methods at?"

I learned from a couple of books; if I looked through my library
I could find them, but you could just search at human kinetics.
There are a couple of different protocols. Contract/Relax,
Contract/Relax/Antagonist Contract, and Spiral/Diagonal

Depending on how deep you want to go you can learn them
all. The CRAC protocol is the best balance of being easy
enough to learn and very effective.

"How long should I hold each stretch for?"

From what I understand, in static stretching you need to hold
for at least 30 seconds.

If you are doing A-I stretching, it is usually 2 x 10 reps for 2
seconds each rep.

Work on dynamic range of motion.

Try Davies hurdle hip mobility drills.

thank you
Coach Hale

15secs or 20 secs for static stuff but I do 10 as well I don't need to go more.

Just work on range.

Warm up: (skip, run a bit etc, get your body warmer a bit and the blood flowing)

Do dynamic stretch's and such.  Swings with your arms, legs, twists with your body.  START SLOW and BUILD UP TO A PACE.

Then do some isolation stuff and PNF stretch's. 

I have a good range in every joint and I do that.  Just work on it.  Once you get a good dynamic range you can kick better but you got to do isolation stuff to do the splits. POINT YOUR TOES when you stretch.  It releaves pressue on your hamstrings so you can stretch more.