How much increased % Elemental damage with attack skills can you have?

My question is, what is cap for having % increased elemental damage with attack skills? Where does your % increase to gear pieces stop increasing your elemental damage with attacks? Thanks.

They don't stop. You can keep stacking it. There is no cap.
You can have 1000% increased damage if you can find the items and passives to support it. People have builds in the hundreds-of-millions of DPS.

What is more important is understanding overall damage calculations.
There's a formula to understanding how to get the most value for your damage, health, etc. This formula applies to almost everything in the game. It goes like this:
Base Value > Added Flat Values > More > Less > Increased > Reduced = Result.
There's probably more to this, but I'm just trying to keep the explanation as simple as I can. There's some VERY detailed guides that you can look at to help explain it all further.

Thanks for your answer, grishnakah! That's exactly what I wanted to know.