how long does it take take to charge a completely empty digital camera battery?

It takes approx. 2 hours to fully recharge a completely exhausted battery at room temperature (23°C / 73°F). The time required to recharge the battery will vary greatly depending on the ambient temperature and the battery's remaining capacity.

impossible to answer as it would depend on the battery type, the battery size, how old it is, the charging rate, etc etc etc.

Depends on the battery, charger and camera.

depends on the charger , the come in various charging time , some take and hour and some take 12 hours

Typically a few hours, if the battery is in good condition. 

On average 2-6 hours.  It could be longer if the battery's capacity is larger or if using a trickle charger.

As others have stated, if a rechargeable battery, like a LiOn, drops below a specific charge, the battery's internal circuitry will prevent it from taking a charge.  There are, however, chargers that are designed to override this and charge up the battery, even if it's completely discharged.

Completely empty? Never!

If the battery is completely discharged, there's a high chance it'll never charge to full capacity ever again. That's if you continued using it for days after the battery indicator said empty already. Ideally, you should never let the battery fall below 10%.

The time requred varies with each individual battery.   

If a battery is discharged too far, it will not ever charge properly again. 

impossible to answer as it would depend on the battery type, the battery size, how old it is, the charging rate, etc etc etc.

If you plug it in and it says it is fully charged in 10 minutes, that battery is shot. But it certainly shouldn't take more than a couple of hours to charge.