First, give the battery voltage time to settle on the sampling capacitors. This is t-sample, which is 40ms minimum for 1uF sampling capacitors.
Then, you must read out the voltages on AOUT after selecting each voltage via SPI. The total time will depend on the speed of the SPI bus.
Lastly, use Figure 5 in the datasheet to help you calculate: t-D_H + tD_LS + n * (time to select Cell n via SPI + t_SET + ADC conversion time), where n is the number of cells