It's too subjective. Factors like vaping frequency, wattage, coil type etc all have an effect on how long a device will last from full charge to discharge. A lot of the smaller pod dont have any indication of battery level either.
Ideally somewhere around 2000mA + would be good but I would say, as long as you know an approx duration that your particular mod will last, you can take another out with you or if it's a replaceable battery, another battery.