When the MAX1896 is turned off, the soft-start capacitor is internally discharged to ground. The capacitor is discharged to ground via the reset switch. The datasheet specifies the maximum on-resistance of the reset switch. This parameter can be used to determine the time-constant for the discharging capacitor. This will give you an idea of how fast the converter will turn-off, since Iout=0A at Vss=0.5V.