I'm hitting the beginning of freeze arrows without problem, but I'm not sure when to release. At the moment when I release I always seem to get an OK. 

 What do you guys do? ... or any tips on how to improve?

I appreciate any advice!  

In most versions of DDR, OK is the best you can get on a freeze.

There's no degree of accuracy for freezes like there is with arrows.  You either got it or you didn't, and an "OK" shows that you got it.

You can actually move your foot just before a freeze ends.  If you have an arrow even with the end of the freeze, you can hit it with the foot that was holding the freeze and not break it.  

Also, you don't need to remove your foot from the arrow when the freeze arrow is over.  As long as you hold it for the length of the arrow you will be okay.

If you hold the freeze arrow the whole time, you get an OK.  If you let go too early, you get a NG.  You can't get penalized for holding it too long, just like how you can't get penalized for stepping where there are no arrows.

You guys are the best! I really appreciate everyone's advice  

 Peace!