The ACHT is the average time (usaully in seconds) or duration of a call, for example, if the average of the calls is 2 minutes, that means your ACHT is 120 seconds, you could measure it by dividing the total traffic by the total number of call attempts or calls. If the traffic is in Erlangs then your ACHT would be in erlangs (i.e. if the ACHT is 0.1 Erlangs that would represent that the lenght of each call in average is 0.1 Er or 6 minutes or 360 seconds, rememebr that 1 Er equals 60 minutes).
The total traffic can be obtained from the statistics of your switch as well as the call attempts.
Be sure that the traffic and the call attempts were obtained during the same period of time (for example a 24 hour period or specific day).