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Not long, although I realize that doesn't help you much. Sometimes when you can see the top of the ingot harden, the inside of the ingot is still liquid. What you need is a heat sink.
When I'm pouring ingots I'm outside, preferably in a shady spot in the yard. I put the ingot mold directly on the soil because the earth is generally a great heat sink. It wicks the heat from the mold fairly quickly. I'll pour an ingot mold, then grab the mold with pliers and flip it over, so the ingots fall on the earth, then flip the mold back over to refill. It's quick.
If it's raining out, and I have to use the garage the molds take longer to cool sufficiently that when I tip the mold over the ingots don't break. Generally a minute or two is sufficient.
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