Hi Terumasa
1. Calibration time: Typically somewhere between 5-10 seconds would be a realistic estimation.
2. When does ADC sample input data?: It is very hard to find the exact time that ADC has sampled the data, but the closest you can get would be to look at the zero-crossing indication of the voltage channel. If you look at the ZXA zero crossing interrupt signal on scope along with the input voltage signal you apply, the group delay in the voltage datapath, shown in figure 51 (page 23) of the datasheet, can be found out. Do you think you can try this experiment?
Regards
hmani