How long does it take for a car battery to explode?

I have a battery in a 1972 cutlass supreme and when i connect the battery, the negative terminal sparks more than it is supposed to. well the negative smokes after a few minutes and there is no power to the car. Its obviously grounding out somewhere. What i want to know is how long it takes for the battery to explode while connected like this? i want to be able to know if i fixed the short but im too afraid to connect the battery because of explosion. thanks!