This will depend on the size of the 12 v battery.
Cannot say as different batteries with the same voltage have different capacity (rated in Ampere Hours). If you had instead asked "How long will it take 532 milliamps to drain a 350 Ampere Hour battery?" that would be much easier (657.89 hours).
3 hours
It depends upon the amp hours of the 6 volt battery. The higher the ampacity of the battery the longer it would take. I hope that this is a hypothetical question because why would one want to fry a battery.
approx 6hrs
Yes
How long does it take for an 18v battery to charge
for a rechargable 9 volt, about 4 hours for full battery.
It takes about 6 hours to charge a 7.4 volt battery when using a 5 volt charge. For the best results try charging it over night.
8 hours
Around 4 hours on a dead battery.
About 2 hours if the battery is 100% discharged.
14 hours approx