It takes two ticks of charcoal - which isn't very long. My guess would be in the neighborhood of 10-12 minutes. Just make sure you have a smithy hammer to process it, and you'll have your wrought iron or your horrible disappointment in next to no time.
I've tried doing this but it keeps coming out as cast iron instantly, or not instantly but after maybe two charcoal lumps. Do I have to keep it going further to make wrought iron?