A completely dead car battery can be recharged as long as the dead car battery doesn't have a dead cell in it.
If the battery has a dead cell or dead cells then the battery won't take a charge and then you won't be able to charge it back up.
The battery should be fully charged within 4 to 8 hours and if the battery still hasn't took a charge after 8 hours then the battery likely has a dead cell and you need to replace the battery.