When a car battery finishes charging how long do I have to unplug the charger before the battery is damaged?

This needs a little explanation since it only applies to a specific type of charger. My charger is a standard vintage Sears 6/2 amp charger . Back in the day the chargers did not automatically shut off when the battery is full. There is a dial gauge on the charger that indicate percentage full.

The problem is that since there is no alarm or autoshut off I have to be Johnny-on-the-spot to shut it off. I assume the battery will be irreparably damaged if I let it overcharge. How long do I have to get to the charger and unplug it when it finishes charging? I can use the 2 amp since I have plenty of time to charge the battery if this would decrease the likelihood.

There is some chance the battery is already fully charged so in this case I would shut if off immediately.