"Assume you are teaching student pilot to fly in a light piston twin - what do you deem to be a suitable asymmetric commital altitude"
As A and C says it will depend on the student. ACH is designed to allow the aircraft to sink whilst going through the go-arround drills. Initially the student will be slow, may make mistakes and will inevitably loose some height.
As the student progresses the drill should be performed in a way that minimises the height loss. In any event the exercise is flown visually at the rating training stage.
"and does it change with time, experience and rating they hold or are training for??"
Yes
When the student reaches the IR skill test level he will be required to demonstrate an approach to minimums on instruments and demonstrate an asymmetric go arround from that height; so ACH becomes the same as the minima used.
Would you go arround from 300, 400, or 500 ft if you have an engine out and a 250ft cloudbase?