The time it takes for a dead car battery to fully recharge depends on how dead the car battery is.
If the battery is just a little dead then it may only take 4 hours but if the car battery is fully discharged then it can take from 8 to 12 hours to fully recharge the car battery.
The battery should be fully charged back up within 12 hours and if not then the car battery likely has a dead cell or dead cells and then battery would need to be replaced.