a top of the range 12 volt fridge drains 20 amps per day a cool box 4-6 amps. You have probably 60-100 in your battery* [fiesta to big diesel as scale]. You need huge cranking amps to start so any voltage drop will be poor for you car at start up. So roughly 5 hours and youcan probably still start the car.I would fit a second battery with split charger personally or ice packs in cooler etc
If you are drivin git is fine to use as the alternator will respond to the additional load and will not flatten the battery.Dont run engine to charge battery as it is really ineffiecient /almost useless as a charger
* It is considered flat when 50% of them are used[to prevent damage to battery as it is not a traction battery you are using] and may start your car but it will struggle.