If the chiller is running, and the purge mode is set to "auto", then the purge will run as long as the chiller is.
If you mean the purge pumpout time in a 24 hour period, I always use more than about 20-30 minutes a day as the cutoff point and think that it is significant enough to perform a leak check on the chiller. (assuming it can be taken down long enough for that)
Unless the chiller is so new it was just started up. Then there could be air introduced during charging. If that is the case give it a day or two (with chiller running) for the purge to remove that air. I'm just curious, but did you ask the Trane tech who started it?