The fastest you can cancel charge punch is seven frames after you input it. You'll know you got the fastest possible charge punch cancel if he does no arm swings and you hear no sound effects. It looks like
this.
As far as your other question, I don't know how many frames DK is in the air for a short hop with no fastfall because that's pretty useless information, but if you want to figure it out yourself, it's just going to be
<Number of frames DK is airborne during a short hop with no fastfall> minus 9 frames
you lose 7 frames doing the turnaround
you lose 2 more frames because you initiated a special attack on the first airborne frame which shortens your jump (read more on that
here. Although the first post mentions only aerials, if you scroll down you'll find the same jump shortening effect applies to special moves too.)
Frame 7 punch turnarounds are ****ing hard to do consisntently because it's a frame perfect input, which are already hard on their own but are made even more difficult because you have to use the trigger to perform the cancel and triggers are a little less responsive than buttons when it comes to quick inputs. You can also cancel the charge punch with Z but the z button also kind of sucks and you'd have to play claw to really make use of it, which IMHO isn't worth it for just one marginally useful tech.
If you're interested in a way to turn around into a jump with more actionable frames, try out a
pivot bair instead. Not only do I find it easier and slightly more versatile than a punch turnaround bair, but learning pivots in general will help out your DK much more than learning how to frame 7 punch turnaround consistently because they can be used for so much more than just turning around into a jump.
