It's supposedly a "just frame", so the window is 1/60th of a second. The timing is right about when Kung Lao grabs the leg and holds it on his side. I don't know how to tell you the exact frame, you'll have to keep trying until you find it.

effffffffff....

thanks for the info though

I totally gave up on that move...

If it is just frame, it's not exact just frame, so you can just rapidly press the button, and it should come out the majority of the time. Hope that helps.

So the move is 2,4, 1+3(also grab) I find it easier to use the grab button. And then the timing i've been practicing seems to work when you press 2 right after he grabs his leg.

As his arm grabs the leg, you input the button press then. The timing is extremely specific and almost not worth the stress. If you watched JWong's Kung Lao in the PDP tourney, even he was missing him more than hitting it. Tekken's just frames are a lot more leniant, such as Paul's Demolition Man. The KL JF just seems a bit ridiculous.

Having said that, mashing it out will sometimes make the move come out. So if you're not looking to learn the timing, mash it out and hope for the best.

I had a lot of trouble with that move as well. I found that when you put in the input Y, B, X+A (I'm guessing that that PS3 input is Triangle, Circle, X+Square) start tapping Y or triangle REALLY fast. It doesn't always work, but it works more than you trying to find the exact input on your own.
The same thing goes for "Uprooting Sweep". Just put int the first input and tap B or Circle really fast. It almost always works. It's an iffy type of move though. I prefer using the "Iron Broom" instead. It's weaker than the other two but still gets a good hit off and is a lot more reliable.