In California summer, and keeping door open/close to minimum I have observed between 18 to 30Ah use in 24 hour period. Check the current battery capacity to see how long the fridge will run.
For standard lead acid we cannot go below 50% rated, for deep discharge maybe 60-75%.
As an example, if standard lead acid capacity is 100Ah then usable capacity is 50 Ah. So you may be able to use the fridge maximum 2 days without starting the vehicle (assuming 24Ah draw in 24hr)
The charging time will be dependent on alternator capacity, if the vehicle is already electrically loaded you may need to keep engine running for 2-3 hours to add 1 more day of fridge use.
I use dual battery setup (LiFePO4 batteries 100Ah), it can be used till 80% capacity so 80AH. This setup is able to run the fridge for 3-4 days without starting the engine. I am able to add more day with 2hrs of engine run. The main car battery is not used, so will not face engine starting issues.